It's not the best to rest the gun on the air tube. It's best to rest it on the fore-end of the stock. But I've seen bi-pod mounts that are designed to clamp onto air tubes and bottles. Lots of guys do it.
One caution is that if your air tube and barrel are connected with a barrel band clamp, then any flexing of the air tube by resting it on a rest, may also flex the barrel, thus influencing your POI. If, however, your barrel is free-floating, then resting on the air tube will have no effect other than changing the harmonics of the gun. This is why it's best to rest the gun on the fore-stock.