Configurable .gitignore filename?

If project or directory belongs to more than one GIT_DIR then some of
the GIT_DIR's may need a different .gitignore files, it would be
useful if one could define it independently, e.g. in GIT_DIR/config:

[core]
ignorefilename = .gitalternateignore

Has anyone considered making the .gitignore filename configurable? It
could be in a config file or env variable, but either way it would be
handy to be configurable. I'm not familiar with git's structure, but
if ignoring is only used in git-add then I suppose I could make own
git-add script if this is not implemented in core git.

I intend to use GIT as a rsync replica (or Dropbox replica) cause it
suits it better than rsync, since GIT doesn't have to check all files
each time I sync (and GIT can contain versions / backups, works better
on Windows and has a simpler needs for the server), more about why I
need this: https://groups.google.com/forum/#!topic/git-users/E-Au9P0A6NA/discussion%5B1-25-false%5D

- Jari