> 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:

https://github.com/domtronn/all-the-icons.el

Check out the screenshot, e.g. the mode line at the bottom which looks quite nice:

https://raw.githubusercontent.com/domtronn/all-the-icons.el/master/logo.png

It would be nice if emacs shipped with these fonts and actually used the icons at various
places like on the screenshot.