Multisite Drupal with Organic Groups problem.. custom /user and /user/register links for each group?
I would probably look at PURL for this, an API to make a part of a URL persistent.
PURL itself does nothing since it's an API, but you can get started with OG Purl.
Be warned, it's more advanced to be creating your own bridge like OG Purl does. A lot of motivation is required :)