1) A lot of companies are paranoid and will want to run this inside their network/firewall. You should develop that option (look at GitHub firewall for example)
2) What incentive do platform providers have in pooling achievements / users - I don't see that happening. I would focus on a white box rewards platform - think SimpleGeo for rewards.
I wouldn't focus on having Webchiever being the destination for leader-boards ...etc because then you are competing with the people you are allegedly providing a service for. Become the plumbing, and if you do it well people will pay.
Cool idea.