Part of the solution to this bug report is to teach LJ::auth_fields_2 that it can still display the dropdown list of the user parameter matches the remote user or some community account the user has access to post in.
I have made a patch to do that. The remainder of the fix is to simply add ?user=$user to the end of the URL for uploadpic.bml in editinfo_do.bml, which I've not submitted a patch for because my goathack account is also currently doing the $LJ::READONLY thing so it's inconvenient.