Feasibility of reverse polarity feature
It would work.

The loss in efficiency would be given by the diode drop divided by the operating voltage.  Obviously, efficiency would be maximised for a low-drop diode, and a high operating voltage. 
 To give a couple of examples:

Using a standard 1n4148 diode, with a forward voltage drop of 0.65V, and a 1.5V battery, efficiency is only 55%.  

Using a 0.4V diode with a 48V lead acid would give 99% efficiency.  

These numbers are to provide protection only - plugging it in backwards wont work but won't damage anything either.  

For it to work both ways, you need two diode drops, and efficiency is 10% and 98% respectively.