brucepick - here is one idea. Why not use voltage booster circuit instead of 1.2V D-cell pack?
AutoSpeed - Voltage Booster, Mark II
Just buy one voltage step-up converter per-headlight (so you dont stress 150W max circuit too much - probably can stay with passive cooling that way too), tune your voltage to 14V and you dont have to deal with Nimh booster pack anymore.