Were you able to get this to work? Did you have to customize the login form webpart? I am starting test and have hit a few road blocks on 8.1.
1) Only /admin (cmsdesk) is redirecting to enter token. Logging into the member site, the MFA is not triggering, it is only denying login.
2) What did you do for the SMS? I was going to write a webservice to send these. I looked for existing SMS Marketplace packages and code, but didn't find anything.