I've done this, and yes it works fine. The only thing I would add is to plug up the shifter cable hole so that water and dirt doesn't get sprayed into the shifter mechanism.

FYI, there are 10 speed mountain cassettes out there, although they are expensive. IRD and SRAM XX or Apex come to mind. Depending on how low you want your gearing, you could also do a 11-28 or 12-27 road cassette and use a MTB crank. STI + a road FD will shift it fine. (Don't use a MTB FD with STI). Or use Tiagra 9 speed. Anyway, there are options out there if you don't like the barcons.