Single up front, gears at rear

They have bolt-in dropouts for ally MTB frames don't they? Get a frame thus equipped and you could possibly get some suitable horizontal dropouts made. If they are not already available to buy

er if you are talking about bolt on mech hangers, a lot of mtb frames have these so that a bent mech or stripped threads is easy to replace, don't know of many with bolt in dropouts, except for dmr swopouts

lots of companies make mtbs with chain tensioners built in (be it track ends, sliding dropouts etc) such as on one, orange, dmr, bianchi