I am not really sure if i like the idea of having pow camps in a game. As a player I may not necessarily want to capitalize from inhumane treatment in pow camps ("hard labor +3"). Especially the treatment of russian pows in german captivity bordered on extermination. We are pretty close actually to asking for the inclusion of concentration camps. Thats just my opinion why I would not support the idea of pow camps in sc2.
If HC should pick that idea up however, some suggestions:
Pows are generated when an infantry or Tank unit is destroyed.
The number of pows generated depends on unit type, unit strenght at the beginning of the turn and random chance.
The total number of pows held captive generates a possibility of partisans appearing in your homeland
But my favority solution regarding pows - do not include it as an aspect of the game!