How to extend UserRecord?
When extending Craft's active record classes (like UserRecord), you'll only be able to setup the AR relationships one way (from your class to Craft's i.e. MyPortal_ProxyUserRecord =&gt; UserRecord) as you've done.  Setting up the relationship in the other direction (UserRecord =&gt; MyPortal_ProxyUserRecord) would require hacking core Craft files (not recommended).

For queries going in that direction, you'd need to go outside the scope of AR and use QueryBuilder to manually build the SQL query you need.

As for your current structure, I'm not sure what the purpose of MyPortal_ProxyUserRecord is.  Why not just have MyPortal_UserProfileRecord and MyPortal_OrganizationProfileRecord link to UserRecord directly? Maybe you just want a single place for that interaction instead of two, I suppose.