Re: I'd love to be able to tighten up the bass of a Custom Custom.
I agree with Jeremy and Bob about shorter screws giving a tighter more focused sound. But the hex screws are a little more effective. The length of the screw is the most important factor, but the materials and shape of the hex screws is also a factor.
You can buy a screw cutter built into a wire stripper at any hardware store for just a few bucks. They are very easy to use and work great...not filing or polishing needed afterward (usually).
Getting hex screws under the lower wound strings, adjusting them higher and lowering the bass side of the pup, and cutting the hex screws (if necessary) so that they don't extend at all past the baseplate should do the trick.
The next thing to try, if that doesn't do enough, would be the UOA5 as suggested earlier.