Chris,
I believe if the plates are on edge, they would have more strength than if laid flat. Laid flat they could flex, but as long as you weld them together they should work, however solid is certainly best. That being said, I was told that a stack of plates wouldn't work for my power hammer anvil due to the bounce factor and I would have a "dead" anvil. Well, stacked plates was what I could manage within the budget I wanted to maintain so it was suggested that I put the stack (18 round discs, 1.25" thick, 8" diameter) under pressure. So, I used a friends 70k # hydraulic press and welded them under pressure. I can't be certain, but I believe my power hammer "thinks" its solid.
With a fly press, you won't be bouncing the dies off your work as you would with a power hammer. If laminated bolsters is what you have to work with, work with them. Improvise, adapt, and overcome!!
