Here's a hacky way to do it:

Provide your profile URL on signup, which would allow your karma to be seen. Then the site would give you a generated code for you to put on your profile.

Once you put the code on your profile, you go to the new site and click "verify" and the site checks your profile for the code.

It ain't pretty, but it'd get the job done.

This doesn't immediately require pg's input - generate a sufficiently unusual string per user and authenticate only the users that have that hash in their HN 'about' page.