Plugins would definitely be the wrong tool for the job for actual new emulator development, but if you needed to do something easy like add an NVRAM for a router rehost you could do that with plugins e.g. by skipping an instruction and using a callback to implement the desired behavior (as long as it is something a plugin is allowed to do -- you can't access device memory yet).
This is actually the whole reason I wrote the patches that allow you to read and write memory and registers. I work on fuzzing, and fuzzing tools are a fragmented ecosystem of QEMU forks and patches that are outdated the moment they are published. Even PANDA from MIT LL which has great support struggled to keep their patches rebased and compatible with QEMU's actually-pretty-fast releases. Upstream or bust, it's really not that hard, it just takes a little persistence (and with LLMs learning git email is easy)!