I wasn't thinking of doubling it up to supplement it. I was thinking more of put it in the front, so it does it's job the way it's supposed to as it.
As a secondary function, it would help size the fit just a bit, instead of me needing that 1/8th inch sweat liner.
I was more looking at it from a, "is this going to hurt the D30 role it's designed for by place it over the already existing 'thin' Rawlings pad?"
There was a very specific mouthguard back in the day, Protec Dent, or something like that, that required the hard teeth background to perform it's action. It had perforated holes in the guard to disperse impact across your jaw. The mouthguard was extremely hard compared to other football mouthguards, the big soft rubber ones, but when a ball would impact it, it spread the impact across a wider area and would keep your teeth in tact. But if the backing surface of it was too soft, it wouldn't perform correctly, and was MORE dangerous than wearing nearly nothing as a guard at all.
Now, understanding that you didn't invent D30, and you don't want to make any official claims...you also know infinitely more about it than most of us, and that opinion is valued.