In the compilation buffer this is only over the file and number. In the
grep buffer it extends to the first match (or something like that).
I don't observe that--can someone provide a test case?
Web page designers often use underline only for mouse over if they
do not like underlined text. It is sometimes awful on web pages
and sometimes instructive enough. Maybe that could be an
alternative here, since the buffers have a fixed structure and if
the underline is seen once when mouse is over them the user
probably would think that clicking mouse-1 will follow the link?
If people generally like this solution, I would not object.