Which terminal should be used for a “C” wire when none are labeled “C”? Older electric furnace/heat pump
According to the schematic, the B terminal does appear to be connected to the transformer.



I'd actually trace the wiring in the unit just to confirm, but based on the schematic it looks good.  

You'll want to make sure the transformer has enough power to supply the additional load of the new thermostat, but typically it isn't a problem.  If you hook up the new thermostat, and the fuse inside the unit blows. That would be an indicator that the thermostat is drawing too much juice.