[Zope] Using eggs in a Zope instance

Python eggs are fun and powerful. However, there is one thing I can't get right when using them for Zope projects: Sometimes I would like eggs to live in $INSTANCE/lib/python instead of site-packages. E.g. if different instances needs different versions of a package. However, easy_install won't install them there, because $INSTANCE/lib/python is not in the PYTHONPATH. And I don't want to extend PYTHONPATH to include all my $INSTANCE/lib/python's -- that would prevent the different version pr. instance approach. How does you egg front runners deal with this? I'm on Windows if that matters. /Anton