Found this when looking for a mod to accomplish the same thing. Too bad it will be an unholy pain in the ... to rebuild my forum and all its customizations for the new version. Shame.
Though for the future, I also say it would be a good core feature to enforce on a group basis, most of my membership's heads would explode if they had it forced on them, but it seems almost necessary for my administration to be secure.
I also would like to request that Yubikey be added to the providers...its what I was looking to use...phones die or you use an insecure web app, but my keys are always with me and plugs right into the PC I'm using. Yubikey is YOTP or HOTP though, not TOTP unless assisted by an app.