Assign User Points in existing website which already contains content
I think User Points Contributed modules is your best alternative available today. I. e. submodule userpoints_retroactive. Excerpt from that project page:


  Award users points for their nodes, comments and votes that they have created so far, before userpoints was enabled on the site.


However, the port of this module to D7 still needs work. Maybe you can/want to somehow contriute to the finishing touch of that issue?

PS: there is also Berdir's Api documentation about this submodule ... (maybe that helps somehow).