John K Jordan
Maybe he should build a frame under the plywood. I've built benches like that with 3/4" ply on top, several layers of MDF below, and still supported on a frame with 2x4s on edge. Maybe a torsion box - he can search Google for torsion box workbench for info.
The local Northern Tool carries some very good industrial casters but I don't know if they have them with jacks. I bought some from Woodcraft years ago for a smaller bench. Step on a lever to jack up or release. Seems like a very heavy bench would need a long foot lever. I have a mobile base for an industrial saw stop saw that uses a foot operated hydraulic jack - will lift 1000 lbs. If handy he could engineer something for his bench.