It is going to increase your electric bill nicely.

You will not only be paying to cool the attic room, you will be paying again to remove even more heat from the return air.

The portable unit will be dumping the heat removed from teh upstaris along with all its operating power (likelyt 50% more than the heat ir removed).

If the room needs 2000 BTU/hr of cooling, the total heat flowing into the return is likely to be at least 5000 BTU/hr.

It my result in inadequate cooling in the rest of the house unless the central air unit is oversize already (especially if the portable unit is larger).

This is forum is really good at keeping you from doing something not so smart. You will be moving heat twice as Brickeeye said. So it will double your electric use for cooling the attic. It would also kill your protable a/c faster since that return vent won't provide enough circulation for the portable a/c.

There is just no way around the window placement ... except a through the wall placement. The outside part needs to be outside.

Now - if you want to try a blower fan at the vents in the attic - go for it (these are fans only that pull extra conditioned air into spaces that don't get enough circulation). These don't do as much as you'd think and rarely work adequately but might be worth a shot.

Best option is to redo the ducting....

Stories like this are way too common. How did these houses get built or areas get finished? Unpermitted? B/C in my area, any finished space needs full HVAC with thermostats on every floor (I think with either separate units or full zoning controls)

Yep, lesson learned. Last night we had both the main AC and the window unit going, with the portable being vented to the return. The AC never kicked into high gear and the entire house stayed warm. Finally around 11PM, I pulled the vent our of the return and rigged into a window. Almost instantly the entire house cooled down.

Thanks for the input. As for the how the house got built this way...I have no idea. It's an older house (built in the 20's) and the entire house is woefully under-insulated and the duct-work is a joke. We're slowly fixing that, but until we do, we're making do with what we have.

I had the exact same idea. Went to home depot and bought a 16x8 register metal cap and a small round metal duct terminated by an intake fitting exactly the end of the portable AC hose. I then cut the metal register plate to fit the round duct. Voila!
My situation is slightly different. In my case I would not be using my central AC at the same time I use the portable. I use the portable AC only when my central ac doesn't work and is scheduled for repair.
Please tell me if it is a good idea at all. I have crank windows and my only other option is to buy a piece of Plexiglass the size of my window's screen and make a hole in it.