> It should be straightforward to take a free font family, strip all
> other glyphs, and modify the remaining symbols to make them fit.
Actually, there is an emacs package already which collects fonts
with free license to provide icons:
Check out the screenshot, e.g. the mode line at the bottom which looks quite nice:
It would be nice if emacs shipped with these fonts and actually used the icons at various
places like on the screenshot.