Duncan supplies #6-32 x 3/4" with stacked Tele bridge pickups, but that leaves two threads to spare or even just one. Doing some measuring, I found 13/16" to be the length needed, but good luck finding that size!
I found an aircraft parts vendor which carries stainless steel #6-32 x 7/8" and sells them in singles. (Try saying it fast.) When placed in the mount holes, 7/8" JUST fits in the pickup rout. Maybe 1/32" from the 'floor'.
Anyway, this solved the issue with my bridge pickup falling off its mount screws. It's a PITA to get that fat wire to behave in that tiny space, though.