IMO, you're better off running the MP-2 off an SLA/RC pack and continuing to run the D100 off internal rechargables.
I would drop the external 4AA rechargable pack and pick up 1800 or 2000 mAH rechargable AAs that will run the D100 for around 4 hours. Get a couple pairs to swap between sets just to be ultra-safe. And for an all day affair, you could push 'em a bit and get a good 6-8 hours off the two pairs.
If you're really stuck on running the D100 off an SLA, I think you could use a voltage regulator to drop the 6v to 5v (close enough to 4.5v it won't cause any damage).