I am actually planning to create a 6 backport of OpenID and Aggregator for 7 (depending on the outcome of http://groups.drupal.org/node/21221 and http://drupal.org/node/293318). I'd use project/openid and project/aggregator for these backports.
Right. That's what I wanted to use, too.
I need to have these modules on 6 to deploy them on production sites - the only way to make sure stuff works. Working on HEAD without deploying changes in months is scary.
See, that's why I campaign for this change.
So let me recap your suggestion, Aggregator DRUPAL-7--1 would be packaged with Drupal 7.x, there would be an Aggregator DRUPAL-7--2 that would contain new features more or less mirroring HEAD.
That entirely depends on the maintainer. Core would come with DRUPAL-7--1 and then what the maintainer does with the project is her/his own doing -- the only two restrictions are that DRUPAL-7--1 should only get bugfixes and DRUPAL-8--1 needs to tag along with HEAD. What you do with DRUPAL-7--2 and/or DRUPAL-7--3 is absolutely your call.
How would Aggregator DRUPAL-7--2 be packaged? It would need a similar infrastructure like a Drupal contrib project, right?
It would be just a package exactly like any other. Already, if I am not mistaken, if you put a module of the same name in sites/all/modules then it will win over the core modules. If that's not so anymore, it'll be too easy to change. Regards NK