Synchronous buck using BJT's in stead of MOSFETS
Current can flow emitter to ground but not from ground to emitter, can
  it ?


Collector and emitter can work in reverse. It's an NPN or a PNP so junctions are somewhat interchangeable but you don't get great β but you do usually get quite low saturation voltages.

On the other hand, the circuit has poor efficiency because the top transistor (NPN) is an emitter follower and will always drop pretty much 0.7 volts or greater unless the base drive is lifted above the incoming power rail.

I would never sanction using this circuit - MOSFETs are far more suited to synchronous buck conversion (generalism alert!).