The SUPDUP protocol is unsuitable for use with proportionally spaced fonts,
multiple fonts, or general bitmap graphics. It is a nice protocol for fixed-pitch,
"matrix of characters" type displays. I think it has been used with proportional
fonts, but you can't really use its editing capabilities in that case.
The Xerox page composition standard (Interpress) is not designed as a dynamic
protocol: it is designed only to express the appearance of a static printed page. I
have this from one of its designers. There might be a way of adapting it to a
dynamic display, but I am inclined to think this would require major changes.
People often make the mistake of confusing keyboard protocols with display
protocols. The link between what combinations of buttons one can depress on a
keyboard and what images one can present on a display is a tenuous one at
best. For example, notions like "meta-bits" make sense only in the keyboard
context, while notions of font and face make sense only in the display context.