Not practical?
The frame on the receiver has to do some things that an auto does not have to. Like hold the barrel firmly in alignment with the cylinder. And the frame gets indirectly hammered when the bullet is forced into the forcing cone of the barrel. That is why steel or alloy frames "stretch" from firing magnums, I believe. Do you trust plastic threads to secure the barrel being slammed with that force? And the frame holds everything in critical alignment. I think that auto plastic frames have a little "flex" in them, and that is what advocates claim soaks up some recoil. I can just see grabbing the end of the barrel and being able to bend it slightly side to side from the thin plastic frame flex. If you add steel inserts strong enough to counter these characteristics, what is the point? Might as well have an alloy frame, to save weight. Due to the different types of stresses, I cannot see it being practical with the current non metallic materials, unless for a low power thing like a rimfire, like someone suggested. Now, there might just possibly be a way to make the grip frame of something other than metal. Like with the synthetic grips, the grip frame of a Ruger that is metal is pretty small. But it would still need to be strong enough to support the mainspring, or have an insert for that. Would certainly shift the weight balance forward!