How about to add No$GBA core (GBA version only)? No$GBA has the support of the solar sensor allows play a Boktai series without patches.
I believe No$GBA is classified as commercial by the author, so that would be a major issue. The author himself would have to show interest in porting it over to Retroarch.
quote from no$gba site
No$gba is now fully freeware
freeware does not mean open source…