Custom footers

[Jérôme Plût]

I don't understand how to define custom footers. Is there a hook in
odd/evenpagefooter where I can plug a Scheme function, or are they
limited to the very poor language of \fromproperty and \on-the-fly?

What I want to do in my case: I have a big PDF file containing all
parts for a big piece. I want the page footer to contain something
like

Title - page 33 - Tenor Trombone (1/3)

OK, so counting the number of pages of an individual part might be bit
optimistic, but hey, TeX can do it, so why not. But I hope to at least
be able to add the local page number.