I've done it! Not quite what you're thinking of though (I dont think you can train out of them what's been bread into them).
I'm in total agreement with everyone thats posted, they hunt for themselves and will range liberally! I've found that our little hound doesnt always scare the birds enough to make them jump, cover or no cover, I'm pretty sure there have been a few coveys of quail that thought, hey we could take him why bother fly'n, but they more or less just move along the ground away from him. Then the baying starts and then the following it for what seems to be miles, my little guy doesnt retrieve though - I leave that job for the big dog. Whatever it is once its in their nose, they aint gonna stop.
I pretty much just run him on grouse now and he does a pretty good job considering he's just not bred for that kind of thing, what I've done to keep him close to the gun, is gotten one of those retractable leashes, but made for a bigger dog so it can take the abuse. I put him in a harness and run my belt through the leash handle/housing and away we go. Just make sure to get a lead that has a coated wire, uncoated and it'll likely frey once you get into the nasty stuff. You'll still tangle up at first, but a little practice and it works fairly well. I can pretty much go on auto pilot after that, once my ears are ringing and belt is tugging it's go time!
It works, but nothing like shooting over my GWP