If you do decide to bond the shielding to the DC supply, just be sure that the negative of the 24V DC system is really the common rail for the instruments.
Some electronic gear uses a split supply (e.g. +/-12V) so that the real DC common "ground" point is neither of the supply wires.
Each 24V PSU is run from a UPS. Is there any sort of capacitive coupling within the PSU tying the DC to the incoming AC ground for RF purposes?
