can signal ground be carried in the connector shell?
The reason why it is not optimal, is explained here as "the Pin 1 problem". Here is an example of what can go wrong:


  Impedance-to-ground is frequency dependent. A good connection in the audio band can be a bad connection in the RF band.
  
  
  
  The black wire takes pin 1 to the body of the mic, and the orange wire takes pin 1 up to the circuit board. At audio frequencies, that works just fine. But at 56 MHz (the frequency of TV channel 2), the inductive reactance of the black wire is about 4Ω. If you try to use this mic in downtown Chicago with a properly wired mic cable, the shield current induced by Channel 2, Channel 5, and a bunch of FM broadcast stations causes enough voltage drop in the black wire (which the orange wire adds to the audio circuitry) that both the video and FM signals are clearly heard!


Funny things happen at high frequencies, and how the cable is shielded will have a large impact on the noise coupled into the signal wires. Sometimes, what looks like a good electrical connection can have too much parasitic losses at high frequencies. Shield connections must be physically as short as possible. Also consider that noise coupled to the shield travels the entire electrical length of it, and can re-radiate elsewhere via antenna action (even inside an enclosure).

Combining shield and signal ground can work, but it will be more noisy - it depends on what level of noise you can tolerate.