You can use a generic polar HR strap (at least with the powertap SL, because i've done it. The customizeable display is nice. For example you can have cadence,power and time all displaying at the same time, which helps doing intervals.

As for cadence measured by hub, I think it's extrapolating from speed,power and torque.I don't know how accurate that would be. It doesn't have a sensor forthe gear you're in.