Giant and Stromer have used inexpensive torque sensors with hub motors, basically simple strain gauges, in both cases fitted into a modified rear dropout. Unusually the Giant Twist used their rear dropout sensor with a front hub motor, but it was praised for how well it worked.Any hub system can use a TS BB & controller kit instead of cadence, the issue why most don't use them is the price at about 2.5 - 3 X that of the best cadence system control kits available which tend to be KT & Lishui.
The version pictured below is the Stromer one: