I have a similar set up and I have doors the open in the center and slide in and out of the way. My cabinets are also full overlay. here is a picture of my set up.
Closed:

Something like a GE Advantium wall oven, which is 30"w and doesn't need a trim kit, only the cabinet opening to be shrunken widthwise to fit a 30"w unit. A Whirlpool-made built-in microwave like the ones they sell at Ikea (also 30" wide with supplied extenders). Also, a rollout drawer microwave (i.e.Sharp), or one hanging over the sink area (weakest solution but it would work).

You have other options. I have listed and described four additional options for you to consider. One option I would not consider is a hinge mechanism in which the door folds down. In this setup it can be awkward placing stuff into the microwave and retrieving hot items while reaching over the door.

1. I do not know the height of the microwaves you are looking at but there are hidden hinge mechanisms that would allow the door to lift up and slide back over the microwave. This would provide access to your microwave while still opening a single door. You would need to make sure there is enough clearance above the microwave for the door. This application is shown in the link below.

https://www.houzz.com/photos/microwave-cabinet-traditional-kitchen-boston-phvw-vp~1203633-Kitchen-Newton-MA-traditional-kitchen-boston

2. You could have two separate doors that open and slide back on both sides of the microwave. This hinge mechanism is like the one above with a different mounting application. At 36" in width you would have enough clearance to utilize this configuaration with the majority of microwaves. The link below shows this setup in use with a TV{{gwi:807}} in a kitchen.

https://www.houzz.com/photos/oak-park-kitchen-and-powder-room-traditional-kitchen-chicago-phvw-vp~1233184-Park-Kitchen---Powder-Room-traditional-kitchen-chicago

3. You could simply break the space up using two doors with more common hinge setups instead of using one door. This would negate the issue of a single heavy door. You can see a similar setup in the link below.

https://www.houzz.com/photos/dallas-greenway-parks-1-kitchen-traditional-kitchen-dallas-phvw-vp~1272465-traditional-kitchen-dallas

4. You could either center the microwave or place it off to one side and as opposed to using the filler that you mention might be awkward you could use the additional space for storage. Depending upon which route you choose there could be cubby type storage or room for something such as a bookshelf. The link below shows the microwave off to one side with books on the other however as I said you could center the microwave and even have built in if you choose to.

https://www.houzz.com/photos/chezerbey-modern-kitchen-seattle-phvw-vp~175039-modern-kitchen-seattle

This post was edited by SaltLife631 on Sun, Nov 3, 13 at 8:21

Thanks all! You've given me a lot to consider. I love this site. :)

Localeater and saltlife, my initial goal was to have the flipper/pocket slides you both show. I have those pics in my Houzz ideabook! But with my full overlay face-frame setup, it's not simple. The standard door is wider than the opening and the interior width, so it can't slide away. Localeater, yours are frameless, right? Mine have a face frame, which adds to the challenge! I could order smaller doors, but I'm afraid they'd look unmatched with the rest of kitchen.

Saltlife, that off-center option is intriguing. The cab is finished, so I could do something like that.

Lee, I did look at things like the speed ovens too, unfortunately they are too tall. My cutout is only 15" high. That's my other annoying constraint...many nice wider MWs are too tall for me.

The option localeater and myself mention can be done however as you alluded to that door or those doors will have to be constructed slightly different from the rest. I am glad scrappy bumped up and old thread for another purpose. Take a look at the thread below. The post illustrates one way to utilize the hide-a-way doors with full overlay cabinets. If you notice the hide-a-way doors are constructed different from the rest of the doors however they still coordinate and work well together.

http://ths.gardenweb.com/forums/load/kitchbath/msg072200047176.html?67

We did a door that flips up. Initially, cabinet maker built it wrong with 2 doors opening to sides. That is what is shown in pic. Don't have pic since door was fixed, but can take one when I get home if you like. The door being inset doesn't bother me.

Is what you mean by lift hinges? If so, I will say that I like mine. I chose it even though it does block the cabinet above. Once I couldn't find a place for a micro drawer, I wanted my microwave hidden. The fact that the open door blocks the upper pantry makes people tend to shut the danged thing when they are done. Victory! ;-)

We have a similar setup as you and we have our microwave behind a roller door. I think the door gives some added visual interest.

Here is a link that might be useful: [Roller Door[(https://www.houzz.com/photos/mascheroni-construction-traditional-kitchen-san-francisco-phvw-vp~92054)