Looking at the Shimano tech Docs, there are 5700 and 5700-A derailleurs, which both come in SS (short cage) and GS (medium cage) versions. The 5700 family supports up to 28T officially, and the 5700-A up to 30T. You can probably exceed this slightly, probably 30T on the 5700 and maybe 32T on the 5700-A, by adjusting the B tension screw, depending on the dropout on your bicycle. It'd probably be better to use a GS (medium cage) derailleur for this sort of setup, unless you're comfortable setting the bike up so that you either have too much chain and start to skip when approaching the small/small, or insufficient chain to shift into the big/big, which aren't really ideal to ride in anyways, although it's certainly possible to forget about this while riding and do bad things to your bike (especially trying to big/big on an insufficient chain). Shimano makes a 12-30 10sp Tiagra cassette now that could be good for this sort of setup.
You can also of course just use a shimano mountain derailleur. You can use a 9sp Shimano mountain derailleur with a 10sp shimano or sram cassette and a 10 sp road chain with the rest of your drivetrain. Don't use a 10sp mountain derailleur, as they have different cable actuation ratios.
Only you can decide if your gearing can get sufficiently low for your needs. If you're a strong rider, carrying very little/lightweight gear, and/or are touring in a relatively flat area, this may be fine.
For comparison, a setup with a 12-30 road cassette on a road derailleur would give you a low gear of 30GI, whereas a mountain derailleur would bring it down to about 25GI, which is close to the stock low gear on a lot of touring bikes with road triples. My 2008 Trek 520 had this sort of gearing before I crashed it.