I can't think you will ever get a MRGB to actually run completely dry for any significant time - too much load and too much heat to dissipate.
The UK mil Sea Kings had an emergency lube system which had an additional sump and used the torqumeter pumps to provide pressure to the top bearings in the MRGB following a double MRGB pump failure or significant oil leak - it certainly gave you more than 30 mins.
Sasless - that S-92 'isolation' concept was flawed since it didn't cover the detachment of the filter due to poor design. Without oil the S-92 MRGB failed at between 11 and 12 minutes.