It’s ridiculously simple:
The extension (1) detects a font-include tag on a page (presumably an <embed> tag), (2) loads the font file from the specified location, (3) temporarily loads the font, (4) continues rendering page as normal. …and (5) some garbage cleanup, to uninstall unused fonts periodically.
Detecting a tag is trivial, loading a font is one command. The only slightly tough part will be integrating the two steps, but that’ll be a good excuse to learn Javascript-to-C interaction, or maybe XPCOM.
I suspect ports to OSX, Linux would be simple too.
I know the page designers would love it, and it wouldn’t take long for every blog to recommending the extension for “optimal viewing.”
Custom fonts on web pages make sense in so many ways: for dingbats and logos, for branding, for preventing the curse of rendered text in images (uncopyable , and unreadable for screen readers).
All the pieces are there, I just have to put them together some day.