@bhargava AFAIK, this is a difficult task to achieve, if not impossible. The REST end points for the Self Registration, especially in OpenAM12 expects an Email address as a required argument. I don’t think it’s possible to meet your requirement without modifying the core functionality of OpenAM.
While OpenAM 13 opens up the possibility of using re-Captcha and KBA for User Self Registration, not sure by disabling the E-mail verification if you’d achieve what desire. Just for your reference, appending below the User Self Service using Rest in OpenAM 13: