I have found in this question (and the doc) that one can use ACCOUNT_SIGNUP_FORM_CLASS to define some custom form field presented during account registration. However, this form does not seem to appear in the flow of using a social account (at least not by default).
Is there a way to have it, or to easily customize some handlers so that I could introduce such step ? Otherwise, this would give a quite non-unified signup process (people using social accounts would have to go and fill the information afterwards in some settings screen, rather than just being asked it once).