BTW, for an alternative load bank, places that surplus (industrial not military, but military places may also have some) electrical/electronic/industrial often have larger electric heaters. Recently I noticed a 15 KW, three phase (208 volt!) heater, with a built in blower. IIRC, they wanted about $250 for it (was almost new) and I should have made them an offer.
The water rheostat idea is interesting. Say three 55 gallon drums, one for each phase, with the drum on the neutral (and grounded, of course) and the center electrode being the phase? It would take a while to boil 150 gallons of water!
I kinda wish I could find an old water heater that had three elements instead of the usual two. With three, a load bank would be easy to do.