Several nations' military R&D programs have tried using steel (or other material) ventilated coverings (hand guards) to cover the gas tubes but, to my knowedge, none resulted in any production (military or commercial) AK rifle variants. I think it was largely decided that the use of synthetic materials with, in some cases, heat shields was preferable to other styles of hand guards.
With that said... I kinda like the ventilated handguard / piston tube arrangement if, for nothing else, it's strentgh and durability. I haven't had much luck with the chu (spelling?) wood found on many Chinese rifles. The chu wood is comparatively delicate and splits easily. The grain isn't very dense and is oriented parallel to the gas piston tube's axis (rather than cross grain) making the handguard's strength suspect.
So far, my "legend" seems to like the ventilated handguard / gas tube assembly. There aren't any issues with extraction, and ejection of spent brass is very consistent. I can't tell any real difference in function or fouling or feel of the rifle using the aftermarket tube compared to the OEM tube with one exception. The steel ventilated covering does get rather warm after a couple hundred rounds. Since it's connected directly to the gas tube, that's no surprise. I'm guessing that's one of the reasons it's never been applied to a commercial or military rifle.