I think you've got the question reversed... I've not used KEGA, but Zsnes now (finally) supports the Arcade VGA. Before it did, it was requested, and the responses from the devs (?) are pretty laughable, and not very forgivable for an emu authors. There are a few voices of reason in there though. Actually, it's not whether the emulator supports the actual card, it's whether the emulator allows you to use all resolutions supported by your card. Not all of them do. Magic Engine 1.0 certainly did not, nor did ZSnes up until somewhat recently (although there was a hacked version).
Soft15khz looks pretty nice. Here is a thread you might find interesting. In it somewhere, a user states that many ATI cards (all?) are capable of 15khz display, and only require a firmware hack. I assume this is what Soft15khz does.
I'm thinking of small-ifying my emulation machine. I'm most likely going to have an extra hard drive and memory that I don't need. I have an extra case as well, so might as well buy a cheap board and CPU. I might try that with a standard NVidia card. It would also allow me to make a dual boot and try some of that 15khz stuff in Linux. Apparently the Linux solution allows for certain refresh modes that the AVGA doesn't support (an issue with MAME). Setup is of course harder.
Right now I want medium resolution games. I have a trisynch monitor. I've only one medium res game (Crackdown). The other one I want (Gain Ground) is freakin' balls expensive. I'd love to play it in MAME though!
I'm aware of Unz... just never gotten around to messing with it.Oh, BTW, UNZ is a nice FM Towns Emu. It works with Soft15khz, and you might want to try it on ArcadeVGA to see if it works.