Pass extra data to oauth signup

I’m trying to validate invitation tokens when my users sign up via oauth. This is an issue for a couple of reasons:

  1. There appears to be no way to differentiate between an oAuth login and an oAuth signup
  2. Related to 1, it seems impossible to pass any custom data into Accounts.validateNewUser or Accounts.onCreateUser when using oAuth.

Has anybody worked out a method to get data into these callbacks when using oAuth? Even connection is unavailable for some reason. I feel like I must be missing something obvious – passing custom data to an oauth signup is something that must be very common.