TX101:
If you point a 300F heat gun at aluminium, it will only get warm. It's the AIR that is 300 degrees. The link you supplied talks about the alloy tank getting to temperatures in excess of 265F. To get alumimuim that hot, you would need at least a large propane torch and even then it probably wouldn't even be enough.
If you leave your cylinder in the back of a car on a very hot day, the air temp would approach 200F - and thats over the entire surface of the cylinder, not just a concentrated part, like with a hot air gun. Aluminium melts at a relatively low temperature compared to other metals (1200F) - but it's an excellent heat sink and draws heat away from a concentrated area very quickly.
In short, I don't think using a hot air gun will damage the tank in any way.
Luxfer states:
Do not
expose any aluminum cylinder to temperatures in excess of 265 degrees F (130 degrees C).
(emphasis added) The standard specifically states expose and does not refer to metal temperature.
Catalina states:
Aluminum cylinders should not be
exposed to elevated temperatures, 350oF (176oC) or greater, or the action of fire for any period of time.
(emphasis added) Again, the standard explicitly states exposed and does not refer to the resulting temperature of the metal. The phrase "action of fire" means exposed to any direct heat source, as opposed to indirect heating such as a closed car on a sunny day.
Gavins' point about BS meters (quoted in your signature line) may be valid, but that doesn't automatically lend credibility to an opinion just because it's contrary. Especially when those expressing those opinions are specialists in being contrary. Please demonstrate that you have some basis for your opinion other than your finely tuned BS detector and that you aren't endangering others.
I'm certainly no authority in metalurgy and am not qualified to have an opinion of my own - which is why I've deferred to the manufacturers (and, for that matter, the CGA and PSI) in this matter. In the interest of furthering the discussion, I would like to ask that you provide us with some information about your background and whether or not you are qualified to an opinion of your own. I'm especially interested in information from you about the effect of transient heat exposure on the strength of wrought aluminum alloys, particularly the alloys used in scuba tanks - the crux of your argument.
A word to other readers: TX101 is right about this much - just because you read it on ScubaBoard doesn't make it true. You should always consider whether or not the advice you read comes from a credible source or is just smoke and might be dangerous.