Well, don't try this out on a valuable part, but I once achieved a parkerization "look-alike" finish by accident.
I had wanted to remove the rust on a couple of screw dies, and dunked them in a little pot with common-or-garden rust remover (which I think is basically a weak phosphoric acid) from the local supermarket - nothing special. Then I forgot about them. When I discovered the pot again, some weeks later, the rust had gone - and the dies had acquired a parkerized-looking surface.
I think this could be a good way to get the buttplate looking parkerized again. And to patch up bad sections on other parts. But I would not recommend bathing them in rust remover and leaving them for weeks, as I did. That might be overdoing it and produce an area with a surface that stands out rather crassly from its surroundings.
A bit of experimentaton on some rusty bits and pieces would be advisable, to get a feeling for the speed of transformation before trying it out of the real gun parts.