I have a P-bass that I have modded a bit. I have added a Jazz pickup, a 3-way switch, and have also swapped out the neck for a bound Jazz neck with block inlays. The usual P-bass pickguard still remains though. I was wanting to get one of those '51 P-bass pickguards like on the Classic Vibe that covers pretty much half of the top on the side where the neck joins. I'd also like to get the metal control plate cover. I realize the holes wouldn't quite line up and/or new ones would need to be drilled, but would this be something worth doing? Or am I just better off sticking with what I have? I'm just looking to do something a bit different. Any suggestions?
I'd stick with what you have. The metal control cavity cover doesn't cover much - you'd have a hell of a time cramming everything in there.

Also, you may find yourself going back to separate volumes or a blend knob. I used to have a 3-way switch on my P/J too, and I swapped it out when I realized I almost always used the bass in the middle position. I like to have just a tiny bit of the J pup on. I prefer the sound of the P, but the extra treble from the J makes it sound nice and bright with I'm slapping or playing with a pick. I can't stand the sound of just a J bridge pup though.