Instead of a trim kit, I plan to have a cabinet front that is open in the center, to the dimensions of the microwave but can be swung out of the way. The microwave will be a countertop version, so not hard wired. No trim kit to remove.

You have to have a legal disconnect. Either you can get to the plug to service the thing or you provide a switch or breaker within sight (or lockable) to allow it to be unpowered while it is being worked on.

My townhouse I lived in a few years back just put a locking tab on the breaker feeding the ovens.

As I understand it, this is an ordinary microwave going on a shelf above the counter. I'm certainly not an expert on US codes, but don't see why it would need anything special in the way of outlets, since it can be serviced by just pulling it out and putting it on the counter. (Well, that's how I used to deal with mine when I had that sort of set up -which passed code inspection where I lived at the time). Just make sure that there's sufficient clearance on the side for the microwave to vent properly.

A non built in microwave with all of the needed space around it will look strange above a built in oven. A trim kit is maybe $160. Buy one.

If you can get to the plug without needing to disturb the finished surface or get into the unit, then the plug can be the disconnect. Yes any easily slid out appliance can generally be disconnected with a plug behind it. More "built-in" things need a provision for an accessible disconnect.

You're putting a cabinet door on, though?

You'll probably be violating the venting requirements of the countertop microwave. It could overheat and cause a fire.

If this is a code requirement, it makes no sense. If it were a built-in the electrical connections would be in the same cabinet. The breaker would be turned off before disconnecting. So, I don't buy the argument that accessibilty of the plug is for servicing. Anyways, who services microwaves?

Regarding the cabinet opening "to the dimensions of the microwave", be sure to plan in the additional clearances required for ventilation.

Most microwaves require 3" on each side, 2 to 3" above the top, and at least 1" to the rear. It will be model/manufacturer-specific and will be in the first few pages of the owner's manual (look for "installation" in the early fine print). So net you will need a cabinet that is 6" wider than the appliance and 2 to 3 inches taller than the overall microwave height.

The code difference between a pluggable appliance and a hardwired appliance is likely even though the two would seem to be similar concepts. There is typically some reason for the code and rather than invite issues with inspectors it's better to follow the code.

The requirement is there because typically the breaker panel will be in the basement or garage, out of sight from the kitchen. If something goes wrong in, say, a bedroom, the person might go down in the basement, see the circuit breaker in the off position and flip it back on while someone is servicing the microwave, without knowing it is for the microwave.

The disconnect has to be within sight of the person performing the maintenance or has to be lockable (as I stated) before. The idea is the servicing guy needs assurance that nobody is turning the power back on. while he's working on it.

Even a MW with a trim kit is not a true built that requires a disconnect. It is still considered a plugged in countertop appliance. It's just easier for everyone concerned if the plug in is in the cabinet above. And it is not a big deal to do. Just like a DW cord passes through a hole in the cabinet to the outlet under the sink. Or the vent hood cord goes through a hole in the cabinet to plug into the outlet above it.

Your wood door idea is not going to work. It's clunky and inelegant. It also won't ventilate correctly.

Even "builtins" can use a cord and plug disconnect as long as it meets the accessibility. My GE Advandiums have all been connected behind the microwave. Even though you have to take the screws out to remove it from the cabinet, it's pretty much a sealed unit and when you open it to service, it can't be in the cabinet anyhow.

Even the blasted EZ Bake lightbulb in the thing has to be replaced by removing the top panel.

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Good to know. Now if my contractor mentions this requirement I won't hold up the "bullsh*t" card as I have with other "requirements."

I agree that a cabinet panel in front of the microwave is probably a bad idea. That is just inviting trouble when somebody operates the microwave with the doors closed. But a tambour door might be an acceptable compromise.

Absolutely nothing heat producing that isn't designed to be enclosed should be enclosed in wood without an electrical cutoff switched outlet that is activated if the door is shut. Too many people want to pretend that a Kitchen isn't a Kitchen. It is. And it's messy and dangerous. It doesn't need to be made more dangerous.

Further even a switch cutoff isn't going to be legal in many circumstances. My Advantiums and the Bosch wall overs won't permit it. They both run the fans to continue to cool things down even after the cooking had ended.