Ask me about my source port
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.