You'll have to vent it somehow. If that's the look you love why not get a chimney hood and tuck the micro somewhere else?

"when has anyone ever needed that? "

You mean the ventilation duct??? You always need access to that. How else would you vent??

FWIW, I would not put a MV in over the range; I dislike the look, it is inconvenient, and the ventilation is poor. And I also would never give up my cabinet over my hood - I keep a LOT in there. I personally don't care for the big chimney hood monstrosities - too dominating. But in your case, if you wanted to eliminate the cabinet, you should do a chimney hood and put your MW elsewhere.

Do it right, or don't do it.

You'd still need access to the space above the microwave for when the microwave dies and needs to be replaced.

I have the OTR advantium. The speed cook features are so annoying that I only use it as a microwave. Have you used the Advantium before?

You have to mount the microwave to both the wall and to the bottom of the cabinet. You have a microwave in the wall now, why are you changing it?

Multiple BAD ideas that are not even physically possible.

An OTR microwave mounts to both a wall bracket (which holds the back) and the cabinet above (which holds the front). Without the front, a 'sheet of plywood' would still need some sort of wall brace (large L brackets or 2x4s at a 45* angle from the front to the rear wall) capable of holding the weight of an Advantium (not light) and whatever you choose to put in it. Otherwise, it's going to twist off the rear bracket and go crashing to the floor.

You probably don't want the 120v Advantium. Doesn't work all that great. 240v is better. I know they used to make a 240v OTR version. I almost bought one open box.

We had a bird build a nest in our OTR microwave vent one winter and the only way to remove it was to go through the vent hood in the cabinet. One of those things I never thought would happen, so you would be surprised at how you may need to access that vent.

You need the structure to sustain the weight of the OTR microwave. It is not just about a vent, most homes do not "vent" outside, they vent to inside that cabinet or whatever... but the cabinet offers a lot of weight carrying ability.

So, the cabinet is needed.

The key will be to figure out how to get/make cabinets that you like.

GL!