Troll (26 inch) and Ogre (29 inch) frames with horizontal dropouts are set up to easily take rohlofs, (with chain) but unless I'm mistaken, with belt applications there are frame stiffness design factors that come into play.
Surly should be able to give you reliable information, have you contacted them?
No Surly frames have been approved by Gates.
https://ww2.gatescarbondrive.com/Man...ers-And-Models
But I do not know how important that is, Thorn has not got any approved frames but they have started selling Nomad Mk III bikes with belt drive and I have not heard of any problems yet.