Just to play devils' advocate....exactly what is the problem?
The last t-stat wire I looked at was rated at 300 v....more than enough. And, wouldn't the outer jacket of the t-stat wire be considered a "raceway," much like the outer jacket of Romex?
Finally, most thermostats are glorified switches, and won't be confused by a little induced voltage, as would a PLC.
On a more sober note, the running of the t-stat wire is a matter of controversy. HVAC guys typically run it either with the line set, or 'free-form' to the unit, where it is provided with a little cord-sized plastic bushing entry. And, as this pic shows, it often is run in the flex with the power wires, for reasons of appearance, protection, and convenience.
I have yet to find a HVAC tech who is either trained or equipped to run flex, pipe, or anything electrical. The local college HVAC curriculum teaches a lot about controls, but nothing about the NEC. As best as I can tell, installers spend most of their apprenticeship bending metal, and techs are focused on superheat.
