The NEC2002 and NEC2005 sections are quite clear in the matter. It more or less says "thou shall not mix wires" when talking about low voltage and line voltage under both 725 for Class 2 and 3 discussions, and 800 for comms wires. LAN wiring sections and methods may also come into play as we give lighting fixtures, security, and access control devices IP addressing in the BAS. There are several exceptions that narrowly bound the field conditions where it might be OK that also require physical barriers and separation distance, and there is also an exception under 725 if Class 1 wiring methods are used, which also is quite clear about minimum wire gauge (16 or 18) and insulation rating (600) - NEC2005 clearly also says de-rating methods shall not be allowed.
Not sure what NEC2008 might have that varies the game, haven't looked yet. But then it would fall back on what version the juristicion State is currently adopting anyway.
If the plant mechanics want you to either deviate from the Code, or exploit a loop hole, I would suggest you get it in writing and signed off by a responsible party at the plant (how often would management back the loop hole in the case of safety standards or process performance "vulnerabilites" ?).
IEEE 518 and 1100 may also be helpful, along with TIA/EIA 862.
