Multi-line font-lock parser

[Teemu Likonen]

I'd like to write font-lock code which highlights the first line that
(1) is non-empty and (2) does not start with a "#" comment character.
This requires some multi-line parsing so plain regular expressions won't
suffice.

I'm too stupid to understand how to implement this kind of parsing. I
can write a code which finds such a line but I don't know how to
integrate it to font-lock. I don't understand the (info "(elisp) Font
Lock Multiline") manual either. So I'd _really_ appreciate if someone
posted an example code here.