Finally sorted out - in case anyone runs into anything similar...

Tried connecting the separate ground to the opposite (white) wire than it was originally jumpered to, but that caused the pickup selector to be bypassed somehow. So I ran a third wire from the pup cover lug to the volume pot and all was right as rain - no hum, and the selector switch works correctly. BTW, I really like this 64 pickup. I had listened to some good youtube demos and liked it the best, and it sounds great in the CV - clearer and punchier than the original pup, with nice neck position warmth.