[EricH]

If they have custom PVC storage tanks on their competition robots, the inspectors should have stopped the inspection on the spot. PVC isn't safe for air storage, and those tanks almost certainly aren't COTS. If it's plastic and COTS with appropriate ratings, that's another thing altogether--it fits the requirements.

<R65> To satisfy multiple constraints associated with safety, consistency, robot inspection, and constructive innovation, no pneumatic parts other than those explicitly permitted by the Pneumatic System Rules may be used on the ROBOT.

<R66> In addition to the items included in the KOP, pneumatic system items specifically permitted on 2011 FRC ROBOTS include the following items. All included items must be “off the shelf” COTS pneumatic devices rated by their manufacturers for working pressure of at least 125psi and used in their original, unaltered condition (except as required for assembly with other components).

Now, on to your question. If it isn't a COTS pneumatic device with appropriate ratings, don't use it. But if you can find a COTS source, and can get the parts in time, go for it (just make sure you have what you need to show that it's COTS if asked).

You may also want to take a quick look at <R67-B>, which spells out allowed modifications to existing parts (of which there aren't a whole lot).