Yes, it can be use!
As a matter of fact, any RJ-45 ICOM microphone can be used with Xiegu G90 transceiver.
Of course, the ICOM remote microphone is using a different serial protocol to communicate with the radio so the function buttons WILL NOT work.
Basic PTT and modulation WILL work.
I also tried with the microphone from other radio, from ic-703 and IT DOES WORK.
This is the proof, in a small video I made this evening after a fellow ham said on FB his combination is not working.
Here are some comments about the Xiegu G90 RJ45 microphone pinout: