@RethinkJeff @malcircuit thinking more, I think you were closer with oath, but it should be a central auth for all forums (probably of that software) and there you can either use oath or forum software specific account, and then choose to access forum with one of your configured identities (or spawn a random one for each forum).

A seamless, frictionless way to access "all" forums would be great.

(Forum admins could choose to support multiple such identity providers)