This is what I use to test my LiteX FPGA based RISC-V development. It works fairly well if you can represent enough of the IP which is why for the opensource RISC-V side of things it works quite well, for ARM I have seen mixed results although as you said there is a lot more emulated. They do have multiple Zephyr examples and I'm sure we could get some NuttX ones in there as well. My C# is really rusty and I have never used it on Linux so I got a little lost in how to extend it.
I have been wanting to get the QEMU stuff in better shape for 64bit to make it a lot easier to test and develop the upper half drivers (especially USB) and the subsystems like networking. Being able to attach real hardware through from the host to NuttX can be really powerful. --Brennan
