Do not see anything that would prohibit this installation per manufacture's guide or ICC evaluation services... an ACT ceiling system is not a concealed location per IFGC definitions.
Also, IFGC doesn't prohibit use of CSST in a plenum.....
CONCEALED LOCATION. A location that cannot be accessed without damaging permanent parts of the building structure or finish surface. Spaces above, below or behind readily removable panels or doors shall not be considered as concealed.
ALSO:
Manufacture's guide may include
TracPipe® CounterStrike® Advantages
csst lightningThere are no additional bonding requirements for TracPipe® CounterStrike® imposed by the manufacturer's installation instructions. With TracPipe® CounterStrike®'s improved properties, TracPipe® CounterStrike® is to be bonded in accordance with current requirements of the National Electrical Code (NFPA 70), and the National Fuel Gas Code (NFPA 54), and with any local requirements that may be in excess of the national codes. This may result in the avoidance of additional bonding costs which are required for conventional CSST.
TracPipe® CounterStrike® is listed by CSA to ANSI LC 1. TracPipe® CounterStrike® meets building code requirements (ASTM E84) with respect to flame spread and smoke density. This permits installation in drop ceilings used as return air plenums, and is UL Listed for 1,2, and 4 hour through penetration firestop systems without removal of the jacket. TracPipe® CounterStrike® is also approved by Factory Mutual as a flexible piping system for flammable gases, based upon its ability to withstand stresses caused by earthquakes