It's quite common to place such a device with it's own overcurrent protection ahead of the main breaker. Yes, the main breaker is the service disconnect, this means you will have to get someone qualified to pull the meter. In many cases, this is the power company themselves (in some cases an electrician can pull the meter, but the power company is still required to come back and reseal it again.

Generally, the solution is to just look at other cues: do you neighbors have power? Are the street lights on? Some transformers have pilot lights (as do cable TV equipment that may be on poles around etc....).

You could also carefully just apply a meter to the incoming lugs.

An electrician could wire a small light bulb and inline fuse across the main lugs for you I suspect.

You could try this

Here is a link that might be useful: Powerback Utility Return

I'll second wirenut's suggestion.

I bought one myself a couple years ago, but unfortunately, my panel (with a built-in meter socket) had no place to get the sensor wrapped around. Just take a look first to see if you can wrap it before the main disconnect or the meter.

That Reliance unit is the only one that my own search has found. Reviews posted online are mixed- as they are for most products. I do have space for the sensor wire.
Good idea about watching for lights at the neighbors but my remote location makes that impractical-- and you had no way to know that until now. It would be a good solution for many situations.
A single pole fuse or breaker off the service entrance conductors before the main breaker would be good-- but quite a hassle to arrange. The recent 36 hour outage would have been the best opportunity as I knew that the outage was due to a downed line on another part of my property and would know when the downed line was being reinstalled. So I may do that if it happens again- which may not ever occur. But it has happened twice in the last 10 years.
I have no other ideas at the moment but am still "contemplating".

Having the meter pulled so you can put one on the mains isn't that unreasonable. In fact, if you have the generator ready to go, you don't even need to lose power to the house for the duration of waiting for the poco to come back and turn it on :)

Pulling the meter is $250.00 whether I do it or the POCO does it. The exception is if an inspection follows and the inspector calls them. Too much hassle for me.
Many years ago I worked 4160 volts in a manhole. This is 120 volts to ground. So with no load on the panel, I will install the conductor with the panel live. In the lug with the 3/0 copper. I know it is 3/0 as I am the one who purchased and installed it. Not by any means suggesting that anyone else work with the equipment energized.
The conductor will be a short piece of #12 stranded with fully insulated 1/4" female spade connector on the loose end so that removing or changing any associated items later (if desired) will be easy.

Some users of the Reliance Alert complain about the sound not being very loud. I suspect that the battery strength determines the sound level. And one would be likely to forget to check the battery regularly if outages are infrequent. A dead battery would render it useless. But the concept is admirable.

Just put in the battery sometime after the power goes off. There is no real need for it when you have normal power.

It sure would be more versatile if it had a remote that could be kept nearby.

One of my goals is to keep all conductors without premises overcurrent protection within the panel. For example. the service entrance conductors are.not premises protected until they get to the main breaker. For the conductor to be plugged into the pigtail mentioned above, one possibility is to use a pigtail lamp socket with a plug fuse for that protection and keep it inside the panel. Not the neatest of setups, but functional and as free of hazard as any other possibility that is on the horizon at the moment. The alarm device will be a bell or buzzer as the panel is in the basement and rather out of sight.

What is the lowest-rated Edison fuse that is available?

I suspect that some very low ratings have been made. I have a few at 10 amperes in my inventory.

Here is a link that might be useful: Fuses

It is great to use what is sitting on the shelf, but the pigtail arrangement will be bulky. You might be considering a transformer to run the bell or buzzer.

You can probably get a pretty compact fuse holder for little money and use a sub-amp fuse along with an AC Sonalert. Some of the latter work at 240V and are really loud. You could probably hear it mounted inside the panel @ 100db(A) and the whole package might fit in a shot glass.

In pondering this problem I've considered panel mounting a neon pilot lamp wired direct to the lugs. As far as fusing it- I would propose to wire the lamp with number 28 AWG or smaller to make it self-fused. By "smaller" I mean magnet wire inside spaghetti.

Otherwise the feed side of the fuse holder would be unfused and any conductor 16 AWG or so would be more of a hazard than unfused 28.

I haven't actually done this, just think it might work. I don't know why panels couldn't have this built in.

Here is a link that might be useful: Neon Pilot Lamp

Because most panels aren't used as retrofit transfer switches.

My transfer switch has pilot lights for both the line and generator side.

Agreed that a transformer and a 24 volt signaling device would have some advantages. 24 volt devices are plentiful. But the transformer would be energized all the time. A 120 volt output affords greater versatility in that a transformer could be mounted outside the panel if desired and downstream of the mandatory switch to be used to silence the device at times of normal power supply. I'm still arguing with myself about some of these final details and trying to think of all the possible drawbacks to any of the possible arrangements.

BD, I think you may have misunderstood my post. I proposed using a 240 or 120V son alert, no transformer. They can be had loud enough to cause hearing damage up close for any period. (I don't know the cost.) The whole package can go inside the panel. An inline fuse or circuit breaker could double as the switch.

And how would one access a switch placed inside the panel? Any circuit breakers are downstream of the main. So how would that work?

Put it in the same place you were going to put the pigtail socket. It will be a lot smaller. I'd drill a hole in the interior cover for a surface-mounted fuse holder or circuit breaker.