The maximum voltage output, no load, of an MTH charger's output for a 3-volt system (2.4 volts), is 3.3 volts. A BCR designed for the 3-volt system is rated to withstand more than that. So it most probably (note I am waffling) would not harm the BCR. But again, why do it?
Put loco on track, turn on power. Wait a minute. Now you've achieved the same result.