KT,
I tried trekking bars for a while, I used a grip shifter with my setup, but I also have internal hubs, so grip shifters are the norm. I ended up not liking the bar, my brother however thought it was a great present and has used it for the last two years. He has thumb shifters, and they work fine with it.

They are commonly used amongst the touring group, check in there and see how they have theirs set up.