Did some testing of various formats.
FON format (older-style bitmap fonts like in older Windows versions), PCF format (Linux, etc.), and BDF format all give the same IME issue on startup. Didn't bother trying scarcer formats like BSD uses and such.
esl::IIMEEventHandler::~IIMEEventHandler()+0xe3e2
FNT format (like Rockbox uses and BMF makes) allow the client to start, but character entry is bugged so it's hard to log in, and even if you succeed...
The UI is completely messed up, I can't even find where to continue since the various buttons are either shrunk or cover the whole screen and overlay each other. At this point you might as well use CTRL+N and CTRL+P when playing.
So it seems to me that Abyss is indeed calling an alternate font rendering routine and not simply replacing the normal TTF font?