Does the bass have a PG? If so you might be able to countersink the other end of the nut/bolt in the underside of the pickguard - if its thick enough.
Hi Derp if you don't want to drill yes, definitely double sided tape will do it & if ever you want to remove it you can do it without leaving any marks just choose a tape that has a mild adhesive, i've done that before with most satisfying results
There are these electronic /appliance/furniture/ feet that you can get at walmart radioshack. They are little rubber squares with adhesive on the back. There are four of the squares on the strip. They make a great thumb rest. Set you back only a couple of bucks and if you don't like them they are removable.
Yes, it's a P-bass, i don't have a bridge pickup, so a thumbrest near the bridge would be very handy.
I used some very thin, high strength double sided tape that worked great. I think it came in the type of tape dispenser that regular cellophane, (Scotch brand) tape comes in. (1/2 inch wide.) I did not keep it on for a long time, so I can't say what the long term effectiveness is, but it stayed put while I was using it. I think I bought it at Home Depot.
You can use a regular thumbrest or make your own. Just bring the thumbrest to your local automotive body shop(where I work). They keep double sided tape by large rolls. If it is good enough to keep moldings on cars for 10-20 years, it should keep a thumbrest on with no problem. I'm sure they'll just give you a small piece. Just make sure the surface you tape is cleaned properly.
Take a thin pick, fold it in half, and slide the point end underneath the pick guard. Works a charm for anchoring near the end of the fingerboard and plucking about 3-4 inches from the end of the fingerboard (95% of my playing). Won't work for plucking near the bridge, however.
That's good but what if he ever wants to remove it? Rubbing alcohol isn't going to touch industrial strength adhesive. Anything stronger than alcohol and you're in the realms of dissolving the finish.
I use this on my Steinberger XL2 because I couldnt find a good place to anchor my thumb on that bass: http://www.bassbuddy.net/ Works great IMO!