I suggest to add support for per-repository per-attribute environmental variables in Portage.
These variables would be used when PORTAGE_REPOSITORIES is not set.

Example of setting of them by user and detection of them by Portage:

$ env \
> PORTAGE_REPOSITORY:gentoo:location=/var/db/repositories/gentoo-cvs \
> PORTAGE_REPOSITORY:gentoo:sync-type=cvs \
> PORTAGE_REPOSITORY:gentoo:sync-uri=:pserver:anonymous@anoncvs.gentoo.org:/var/cvsroot \
> python -c 'import os, pprint; pprint.pprint([x for x in os.environ.items() if x[0].startswith("PORTAGE_REPOSITORY:")])'
[('PORTAGE_REPOSITORY:gentoo:sync-type', 'cvs'),
 ('PORTAGE_REPOSITORY:gentoo:location', '/var/db/repositories/gentoo-cvs'),
 ('PORTAGE_REPOSITORY:gentoo:sync-uri',
  ':pserver:anonymous@anoncvs.gentoo.org:/var/cvsroot')]

A separator between components of names of these variables cannot be any character valid in names of repositories.

--
Arfrever Frehtes Taifersar Arahesis