Yeah, a switch in the cable would work but you have to use a stereo cable to switch the "ring" contact to ground to turn the bass on and then open that connection to turn it off. It would not be difficult to make one. Turning the preamp on and off may also make a loud pop in the PA, I've never tried this so I don't know for sure. I can tell you right now that people will forget to turn that switch off....
If your bass players make enough loud pops unplugging the bass the sound people will eventually learn to kill the inputs on the board at the end of services and rehearsals. In my church we all supply our own instruments so the sound crew has to remember to do this for a bass, a guitar, and sometimes a second guitar. Plus, one of our bassists will unplug his bass and take it with him right before the sermon in the second service if there are no songs scheduled after the sermon that morning and the sound crew really has to be on the ball with him! It is worth it, he is a GREAT bassist!!
It is not hard to put an on/off switch on the bass either. The negative lead for the battery runs over to the "ring" contact terminal on the output jack. Cut that wire, solder in an on/off miniature toggle switch and you can turn the bass off. You will need to find space on the control panel where the switch will fit and then drill a hole so you can mount it there. As noted above the bass may or may not pop when you turn it on and off.
You could also put a kill switch (one that would ground or unground the center lead) in the instrument cable, or make a kill switch stomp box to put in line with it. This would allow you to plug/unplug the cable without a pop. I don't use stomp boxes or pedals so I am unfamiliar with what is out there, chances are someone makes one you could just buy.
Another approach that would work well in this situation is to remove the battery from the bass and use an external power supply to feed it. You would need to short out the battery connection with another connector you can buy at Radio Shack. Twist the new connectors leads together to short them, then solder them and tape them up to insulate them. Now plug this into the bass in place of the battery. Now you can feed +9V into the ring contact on the bass output jack to power the bass. Use a stereo cable between the bass and a power supply box you will build (though again perhaps someone makes one you could buy). You can probably just leave the bass on all the time if that is what you prefer to do, or power it up and down with the rest of the PA system.
Whew, the Neutrik silent plug is sounding more and more like a winner after all that, eh?
However, I do notice this warning at the bottom of the page describing that plug "Please note that the signal isn't muted with some types of active electric guitars which are equipped with tip-ring-sleeve jacks that tap to activate the guitar power supply." Not sure why that would be true but if Neutrik sees this often enough to mention it on their web page the plug may not work for you. Bummer!