Warning! I'm not responsible if you screw up your bass or if stigmata suddenly appear because you've run a drill press though your hand. If you don't know what you're doing, and how to do it safely, don't do it.
Here's what I did. First I replaced the aluminum bridge with a maple bridge. I'm told that a 1/4 size bass bridge will fit, but I cut my own from rock maple. If you can't find a piece of quarter-sawn maple, at least make sure the grain runs the width of the bridge, not top to bottom. Trace the outline of the aluminum bridge onto the maple, cut it out, and form the bridge blank into a wedge. (If you don't know anything about woodworking, find someone that does; my intention here is not to delve into bridgemaking, which I don't know much about anyway.) Cut the scrollwork into the bridge as well, so that you have the two slots on the side common to violin-family bridges. (I cut mine with a long, even slot, unlike some bass bridges.)
Now here's the cool part. Go to Radio Shack. (I know, I don't like Radio Shack, either . . .) Buy a Miniature Piezo Buzzer Element, part no. 273-064. It'll cost all of $1.79 or so. You'll also want to get a mono output jack and some shielded wire if you don't already have some of that stuff lying about at home. Carefully pry open the plastic casing. Inside you'll see a brass disk with the piezo crystal on it and with three wires coming from it. Remove the feedback wire (blue). Also carefully scrape off the white ring of silicone that glued the element into the casing. Solder the red wire to the center conductor of one end of a short length of shielded cable. Solder the black wire to the shield of the shielded cable. At the other end of the cable, solder the center conductor to the tip connection of the mono jack and the shield to the sleeve.
Now you want to make a sort of piezo sandwich by putting the element between two thin slices of cork. I found that carefully slicing sections from the cork from a chianti bottle with a razor gave me round sections of the right size. I then sanded them smooth and to the right thickness (determined by trial and error). Take the element, sandwiched between two slices of cork, and push it into the slot on one side of the maple bridge. The fit should be rather tight. Fashion some way to secure the output jack to the strings below the tailpiece a la Fishman (my temporary arrangement was to put a large paper clip in front of the strings and another behind the strings, pushing the jack through the center of them and screwing the nut down in front; I'll make a more permanent arrangement later).
Now it's time to plug the bass in. Be careful! I found my pickup to be much louder than my Precision Bass, so turn the amp volume down before you start. You may have to reposition the element in the slot a few times to find where you get the best response. You should be rewarded with a bass sound that puts the stock pickup to shame.
My next step will be to add a volume control and maybe (haven't decided yet) either active or passive tone control. If you've done this sort of thing and can refine the procedure or offer other options, please email me at bluemuse@aol.com.