I think the biggest GX2 experts we have are either Gary or AboodXD, but I'm not sure if either of them are particularly interested in contributing to RetroArch, they've always got a lot of their own projects in the works.
Core selection is based on RetroArch .info
files, which are just little text files listing what type of ROMs each core can handle and stuff like that. Since the Unai build isn't an official part of RetroArch (yet), there's no .info
file for that build. Personally, I just copied the one for the Peops build and edited it slightly. I've attached mine to this post if you just want something you can download and drop on the SD card (after extracting the ZIP).
Whether you're doing it yourself or just extracting mine, copy that file pcsx_rearmed_unai_libretro.info
into the RetroArch core info directory. If you haven't messed with the directory settings at all, that will be in SD://retroarch/cores/info
.