I also join the first question of Retrofan. Graffitti modes are implemented on indy ECS where they have little use. If it was implemented in indivision AGA we could do MAC emulation on a chunky 640x480 display fast as hell. And with the excellent image quality of the indivision AGA... I have an original graffitti board, and the speed is blazing fast on 256 colors.
AFAIK it's absolutely possible to do this only upgrading the ┐firmware? of the fpga. And it was a promised feature from the beggining. ┐Will I.C. add it some day? I'm sure that if they do it coders will create more software for the graffitti, like a quake version. And this will create more sales.
It would be a smart move in my opinion