Take a look at SPI ( Sensor Products Inc.) out of New Jersey.
They have a film that you may be able to use.

You are misguided in thinking that you want to measure pressure. The only thing you can reliably measure is the force that the clamping mechanism applies. Some type of spring scale would work, or you can realize that the frame of the press is, in fact, a spring that deflects under load. So a dial indicator can be set-up in a way that measures this deflection, which can be calibrated to read the force.

Actual pressure is a distribution across the surface and depends on what is between the plates, and it's uniformity in thickness and hardness.

Re: David's suggestion. @Composite: that's what the film would do (indicate the distribution).

At a previous job we used pressure sensitive film (Fujifilm) to measure contact pressure for knee prosthesis as that is a very significant contribution to polyethylene wear. I was not involved so that's literally the limit of my exposure (heh) to that application. However, if the 'instrument' needs to withstand elevated temperatures I do know if that is the right product for the application(if this website is any indication).

Strain gages.
Walt