fsan:
Safety is probably not an issue if you are talking about say, 3/8 tubing, the conduit temperature doesn't exceed the tolerance of the plastic, and there are no tubing connections near live parts.
However, 2002 NEC 300.11 (2)(B) states, "Raceways Used as Means of Support. Raceways shall only be used as a means of support for other raceways, cables, or nonelectric equipment under the following conditions:
(1) Where the raceway or means of support is identified for the purpose; or
(2) Where the raceway contains power supply conductors for electrically controlled equipment and is used to support Class 2 circuit conductors or cables that are solely for the purpose of connection to the equipment control circuits; or
(3) Where the raceway is used to support boxes or conduit bodies in accordance with 314.23 or to support luminaires (fixtures) in accordance with 410.16(F)"
Your conduit probably does not meet any of these conditions.
