Daniel Jalkut recently wrote this piece about the Touch Bar coming to an Apple external keyboard. That would allow for the Touch Bar to be used with desktop Macs and MacBooks used in clamshell mode. Daniel thinks they’ll eventually get there.

“I find it impossible to believe that Apple would go to all this work, both on the Touch Bar itself, and across the entire range of its own apps and OS features, unless it had a grand vision for the Touch Bar that extends way beyond the internal keyboard of its premium notebook computers.”

— Daniel Jalkut

I agree. I'm certain it would need more charging (and perhaps a bigger battery) than the current external keyboard and I’m guessing it will be pretty pricey but I fully expect a Touch Bar enabled external keyboard to eventually show up from Apple.