Can batteries in series be reverse charged if the circuit is open?
NiMH cells in series will be fine. Self discharge is a chemical reaction within the cell, and does not depend on the internal resistance (although they are somewhat correlated, especially at end of charge). So, polarity reversal can occur only if cells are connected in parallel.

Self discharge is also somewhat self limiting. It reduces as charge falls, due to electrode passivation (which also contributes to increasing internal resistance).

Easy precautions and good practice:


Make sure cells are same type and from same manufacturer.
State of charge is similar when installed, and none are close to depletion. 
Most importantly, charge each cell individually.