This keeps coming up.
Application notes for knock control (Megasquirt, I think) I read once - they advised against mounting the sensor high on the cylinders.
So, yeh, no idea why the factory used a cylinder bridge on the 964's onwards - and it really doesn't seem to be necessary. Lot of guys running knock control without that.
Mine is mounted on an M8 stud under the shroud; others have mounted on the case in a convenient location (like the unused factory tapping by the oil chimney) and report that it works fine there too. Jake Raby uses the bell housing on Type IVs.
http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/1018043-placement-knock-sensor.html
Just need to mount to the case, and the Bosch app note says that ONLY the sleeve in the sensor should contact the case.
I have used a J & S Safeguard for 10's of 1000's of miles on a 930; both the traditional model for CIS/distributor and more recently the Interceptor direct "smart coil" version with MoTeC. Works great. The only false knock indications I've ever seen is from valve train noise when shifting from WOT (it "hears" all the slack when you lift, but clears instantly when power re-applied) or transmission clanking.
When my distributor abruptly froze fully advanced, the only indication I had was the Safeguard lighting up and protecting the motor:
http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/611409-my-j-s-safeguard-paid-itself-last-week.html
When we went motec, my wrench (motec reseller and trained/licensed to sell the knock feature) said "go with another Safeguard; it's so much simpler to setup and use".
The Safeguard model I use now with COP (Interceptor)
J&S Eight Channel Interceptor has per-cylinder knock indication, monitor/headphone output and the ability to feed knock indication back to the Motec.
But since twin-plugged and EFI, the only time it triggered was when I got 3rd at low speed thinking it was 1st and the transmission made clanking noises...