> A single `C-g' wouldn't be quite the right thing, because the user may
> hit `C-g' by accident while font-lock happens to run (and it's not
> hanging), which is why I though a special key sequence like `C-g C-g
> C-g' might make sense.

That one is too fragile I agree. If "special" means `C-g C-g C-g' then
that is fine in my book. If something gets stuck and doesn't respond
to one "abort this" command, I usually tend to try several times
anyway, and suspect this is the same for a lot of people.

> Doesn't seem to have any effect here...

It seems to have some effect with `C-g' after some time.  But still
doesn't abort it, only shows some warning, yet doesn't make Emacs
responsive.

Paul