Vibert boxes were the rage a couple of decades back, and they were basically what you suggest: kind of an in-stream rearing station. I believe they are still used; I recall hearing about some SRC transplanting efforts using such boxes.

The process that I recall is that the fish are captured, spawned artificially, and then the fertilized eggs were placed in the boxes.

The only problem I see is that this method doesn't provide any natural selection for spawning success. The fish have made it to the stream, but the only way you can prove they can successfully spawn in the wild is to let them get their business done. However, for short-term supplementation of runs, I think it sounds pretty good. This might well be one of the proposals on the hatchery reform program, but I'm not privy to that info.

Keith