We're looking for someone to code a UC module (D6) to allow purchasing of access to an OG group. No, not via roles, since we're talking about many dozens of groups. Upon creating/editing a product, the administrator should be able to add one or more groups (via the Features in UC, I would assume) that the user would become a member of upon successful checkout with that product.
Code should of course be contributed back to the community 
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After reading your post, I started writing this module.
When installed, at the edit of a product node, you will find a local task 'Group subscribe'. You can select the groups you want the member to be subscribed to after purchasing this product.
With conditional actions, you have to create a new predicate with trigger 'Customer completes checkout' and action 'Subscribe to the groups ordered'. When a customer completes the checkout of the product, he will be member of the groups selected.
Let me know if this is ok for you or you want something extra.
Greetz
Dieter