dynamic completion-ignored-extensions

[Kevin Ryde]

Is there a way to have a kind of dynamic completion-ignored-extensions
so that for instance a .c file is ignored if there's a .xs of the same
name?  Or even a kind of programmatic ignore so Makefile is not offered
if it seems to be in a build directory of some flavour (and therefore a
generated file).

I tried sticking a defadvice on file-name-completion, but in emacs22 it
doesn't seem to be reached.  I guess read-file-name is C code and
bypasses advice, or something.  Is there another filename reader that
could be could be easily pressed into service and customized?