How could I redirect a successful registration to a standalone homepage page instead of a G2 page?

I can re-direct a LOGIN to a different standalone page in my embedded app without going to a G2 page, however, I can't do this for registration.

What I'd like to achieve is:
- user submits details in the G2 registration page
- if the registration is successful, send the user to a non G2 page

I've tried changing "g2_return" and "g2_formUrl" to include the URL to the new page but all I get is: "Authentication Failure" and "Your change cannot be completed due to a loss of session data. Please try again. If it still doesn't work, try logging out and logging back in."