Don't try this at home - by blanking the pressure relieve valve and adding more weights to the top of the pressure cooker you could create your situation for real, but the pressure cooker WILL explode at some point producing LETHAL metal fragments and LETHAL superheated steam. Your kitchen walls are not built to withstand this sort of explosion and would probably collapse. THis is why pressure cookers have TWO safety systems.
Actually that does not happen!
Firstly they have a little rubber bung with a metal dongle in the middly which moves to release excess pressure.
Secondly even if it did not come out the the closing lugs bend under excessive pressure to allow the seal to leak and release the pressure. The bottom usually bends outwards a little too to create a domed effect.
Tony