lol.. so you don't have any experience of plate failures then Phil..
Seriously, provided the plate is suitably prepared, it will behave like an extension to the block - make the plate locate on the block dowels snugly, circular hole on one which is a good fit, and slightly elongated on the other to allow for the small amount of expansion differential between the iron block and steel plate.. this will stop the plate being able to move around.
The plate face and block face need to be flat, so u get an intimate mating surface, the sealing issue really comes down to keeping the fluids in then, the pressure physically cannot "leak" because it's got nowhere to go as the gaps are miniscule if the surfaces are an intimate fit.
The gasket then sits between the plate and the head, and behaves exactly as it would in a normal setup.
The main issue with the plate is that it makes the combustion chamber shape less than ideal, removing the effective squish area wher the piston doesn't get close to the head, but this is no different to skimming the piston..