with a normal jaguar, the spare leg on the output jack would have a black wire leading from it, and grounded to the brass sheilding plate. since yours probably doesn't have sheilding plates, that afforementioned leg is useless and remains unused.
i can't really imagine the guitar coming out of the factory, and not being grounded. on vintage and japanese jaguars, the strings are grounded by one single strip of unshieded wire, wedged underneath the control plate that has the pots on. then it leads to the lower bridge bushing, where it is wedged underneath that. the wire is pointed out in this picture:
http://fenderjaguar.net/images/bridgewire.jpg
also, you talk about "hum". 60 cycle hum is interference from other electical items. and takes place with single coil pickups. not humbuckers, or opposing single coils wired up in paralell. so we're not talking about hum, we're talking about buzz, when we say a guitar isn't grounded properly, or isn't being touched. when a metal part grounded into the circiut on a guitar is not touched by a human being, it will buzz. because there is nothing to dissipate it
