Yes, you could use a bench supply to run the camera, if you can cobble up a way to attach the leads up inside the battery bay.
The AC adapter uses a wired dummy battery case to connect to the camera. Good luck finding one of those. I have one, but have never seen another with a camera for sale.
Oh, and the dummy battery has a switch on it for ROS cleaning mode, too. I've never opened the thing up to see just what they do in there to tell the camera it's supposed to leave the shutter open. Probably a diode gets switched in, I suppose.
I think I have a dead E3 NiMH battery pack laying on the shelf, though. I got it with one of my parts cameras without a charger. Since I had a few of the older E2 NiCd packs and a charger already, I never did bother to re-cell the E3 pack. That'd do, wouldn't it? You'd have to get it re-celled and then buy a D1 charger, but it'd work.