tips to get it back to factory straightness are still welcome.
I've got some tips that work with vinyl, may work with a pickguard too.
Slow but safe method:
1) Lay it on a flat surface.
2) Put a dictionary on top of it.
3) Then put a dictionary on top of that.
4) Repeat number 3 until you're out of books or lose balance.
5) Wait for days, weeks or months.
6) Retrieve flat(ter) pickguard.
Quick but unsafe method:
1) Place baking paper on a flat surface.
1) Lay the pickguard on the baking paper.
2) Cover the pickguard with another sheet of baking paper.
3) Put a piece of fabric on top, fine cotton should work.
4) Power up a clothes iron. Low or medium heat.
5) I think you can guess the rest.

I'm not taking responsibility for even suggesting it. Just don't overdo it.
I think this pg bending may haveto do with the wood working. If the wood of a bass shrinks a bit this could squeeze the pg, forcing it up.
Sounds plausible enough... or if the PG hole pattern starts out slightly bigger than the body wood pattern, but why would that be? Aha, maybe the PG holes were drilled made in a relatively cold environment? I believe some plastics shrink/grow with temperature changes. But at this point I'm merely speculating.