Uhhh I believe Plus++ is already set up and ready for it. And also it's actively developed by @TheGeneral so I'd recommend going for that. I believe it's on the github repo. You can click
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to view the development thread and it also contains the link to the github rep. Or you could always use a normal plus emu and port it to Nitro.
Yes Plus++ is nitro compatible. Just keep in mind that you have the correct header file named matching your nitro revision / version as they dont send a protocol version but rather a nitro build version in the 4000 packet.