Sign in with openID/oAuth
Since you need oAuth for so many things, perhaps ask them straight off the bat?
PS: Prefill my email below this edit box - I am already signed in to you
We agree this would be a better user experience and fits the model very well.
Unless we have missed something, there does not appear to be a way to prefill the email address in the User Voice widget. They have a suggestion open for it here: http://feedback.uservoice.com/forums/1-general-feedback/suggestions/3325030-be-able-to-pre-fill-email-address-on-contact-supp