If I am not mistaken the hydraulic pump is drive by the main gearbox so if the blades stop turning then so does the pump. I think there may be an accumulator in the system but that will supply limited volume only. It is not permissible to stop the blades with the rotor brake with engines running. In fact I don't know if it is actually strong enough to do so in the first place and as stated above it wouldn't help you since the hyd pump is driven off the main gbox anyway. That being said you still can move the cyclic and collective without the hydraulics but it requires a bit more effort. Depending on how much you need to move the controls it may still be possible without hydraulics.
Hope it helps
Max