Found some more links on Hydraulic Hybrids:

An Australian company that makes an add-on kit for medium and heavy duty military and commercial trucks:
Permo-Drive

Pretty slick looking installation. It looks like it just cuts the driveshaft with their pump and the accumulators.

They are teamed up with Dana to provide this system to the US Army for FMTV's:
Dana Corporation, Permo-Drive Technologies Ltd. Sign Agreement For Fuel-Saving Hybrid Vehicle System

I'm guessing that they feel that they can protect the accumulators adequately enough from hostile fire to justify the gains in fuel economy and added acceleration.

-Jim