It's understandable that you're against per-site specialisation. Hence my suggestion that an option pop up for that site when already crossposting to it. In which case it's clearly special in the poster's network already.

This comment gives an example of what casual LJ commenters do when confronted with "OpenID" (which sounds like someone selling them something) and "identity URL" (what the what?)

Solutions should be generalisable, which the OpenID plumbing is. However, exposing the naked plumbing is failing to work for the people it's supposed to, i.e. the remote commenters.

Whatever suggestion you go with - please, please do usability testing with non-geeks. It's painfully clear that no-one did such testing with the present option for LJ-based commenters.