I can't imagine any locking casters being stable enough even for a solid, heavy workbench. They'd always be wanting to move the bench around, especially when trying to plane by hand.
I have two ideas; but no specifics:
#1 - a lifting caster system like is on the Rigid tablesaw. Lifts with a foot pedal, lowers onto the legs. Might be difficult (and $$$) to find proper hardware enough to lift a workbench, though.
#2 - A wheel set on one end of the legs and a set of lifting jack arms at the other. Kinda like a wheelbarrow setup. Set the wheels on an axle on the outside of the legs, flush with the floor/bottom of the legs. Use a set of removable jack arms on the other to lift and move around. Got to be better than sliding the whole bench across the floor.
Dave