I just saw this last night. Made for a strat, but it eliminates single coil hum for strats. http://www.suhrguitars.com/BPSSC_Flyer.pdf And it works! Check out the first video on this page: http://www.prosoundcommunications.com/english/video/scott_henderson2/index.html I'm not sure exactly how it works, I'm assuming dummy coils of some sort, but maybe something like this could be built into a Jazz Bass pickguard. Would you be interested?