Is there a way for me to see what a certain user's steamid is after they linked their steamaccount to my website? If I am able to get the steamid64 which gets sent as a response over openid I would be able to turn my website into a hub for all my gameservers where players can see all kinds of stats and achievements they've acuired on my servers on the website as well.
Your service already allowed me to make it possible to force my members to log in with their steam account, but it would be more awesome if I could actually make use of this information other than just the quick registration and spam prevention. If it's not possible, see it as a suggestion to make your product even better.
I think the best way to store this in vanilla would be a (seperate) table in the database with the information you supplied. I already noticed that you generate a record in the gdn_oasl_identity table where you link the identity (PK) with the user (FK). Perhaps it would be an idea to add a column called: provider_primary_identity where it stores the primary identifier for that particular provider, if any. In most cases this information will be publicly accessable. When it isn't you could always fill this field with null.
This way I can access the primary identifier for each service in my own database without making thousands, or in some other cases ten-thousands, of API calls per day.