Steve,
GF are excellent swimmers. They like to swim from one end of a tank to the other, especially feeders and other flat bodied GF. For the psychological well being of the fish, I would not put any into a container of such small diameter. JME, though.
As for the burnt wood thing, all I can think of is carbon residue. But what that means for keeping fish, I don't know how that would affect them.
Tannins reduce pH. If you are already starting out with low pH water, then you will certainly have to compensate for these two factors somehow.
What kind of metal will you be using in the barrel? Anything that would oxidize, like iron, would be a bit dangerous. Even copper would oxidize and you'd have too much copper in the system. Don't really know the composition of brass, or how or if it would oxidize.
Anyway, all these points are moot given the fact that the barrel is still too small for GF, ime, over the long term.
Nice idea, though. You could still have a lovely water feature without fish, maybe even with water plants, without all the hassle of worrying about metals, tannins, etc. in that great looking barrel
