Not sure exactly how you plan to use this, but if it were me, I'm adamant about using what already exists. For example, for console characters, I'd probably use a standard texture. Each patch could be a character. Yes, you'd have to have code that renders to a block buffer for run-time display, but, you'd have your headers, it'd be in a standard format, no decompression time, and only minimal extraction time. As a bonus, you get proportional fonts, if you desire (not so good for a console, though, heh :)
Of course, make whatever format you wish, it's your port. But if the goal is for easy end-user modification, you'll have to consider that too. No one wants to "load raw" if they don't have to - doesn't matter how easy it is. Just my 2 cents.
EDIT: Actually, having an icon-like, multi-patch drawing gizmo would be kinda neat, and useful. Things like HUD icons, number displays, Heretic-like inventory pics, splats, etc. Could be useful to shove all of those into a texture... With the right set of extraction/drawing support functions, that could be very useful, indeed.