I've put thumbies on a flat bar before, but I was dissatisfied with it because of the real estate they claim. They fight for space with anything else you might want to put on your bar - cyclometer, handlebar bag mount, light mount, etc. - and the lever swing robs some of the space you might like to use for your hands. You might want to consider one of two options:
1. Stem shifters *gasp* *crickets* Stem shifter mounts are just a band with shifter bosses, it is even possible to get them to work with indexed shifters. If your stem is bigger than the old-fashioned 22.2mm, you may not find a stem shifter mount to fit.

2. Abandon the flat bar altogether and go to something like a moustache bar. You keep your existing brake levers and bar end shifters, and maintain multiple hand positions. If you aren't opposed to replacing the brake levers and shifters, you could use a *gasp* flat bar with bar end grips. I've toured with this setup before and was quite happy.

Personally I don't care much for stem shifters, it's the little bit of elitism I allow myself to entertain. But I've been considering an alternative handlebar for my own bike.