Tassie Wargamer, that makes two of us! I'd like to do the 1444 campaign in 25/28mm. I've been watching the plastic figures, hoping that with swappable heads and other components something might be possible. It may be possible, but I haven't felt confident enough to invest money in it yet.
As YogiBearMinis points out, there are some Eastern European figures that might be of use, particularly as the Byzantines hired mercenaries of different sorts. Old Glory's stradiots link come to mind here for use as Byzantine light cavalry, though probably not all of them are suitable. The guy in the top hat, for instance.
Gripping Beast looked into really late Byzantines at one time, but decided not to. Perhaps they could be convinced.
Eureka no longer does their 100 Club, though the Steve Barber idea is a possibility.
Several years ago, somebody here on TMP was building a 25/28mm 15th Century Byzantine army, and posting pictures of it--very nice pictures, in fact. Years before that, I read about somebody who did the 1452 Siege of Constantinople as a game, but never saw any photos.
For what it's worth, Essex did bring out some suitable figures in 15mm. I have some, but haven't been able to drum up the enthusiasm to paint them.
Grelber
If something ever turns up, perhaps we could meet halfway between Tasmania and Colorado for a Byzantine civil war game! 8^)