The only way I can see possible to oversmoke anything is to use too strong a wood on too light a meat. Like smoking chicken or fish with mesquite. Even then the 'smoke' never penetrates deeper into the meat than maybe an 1/8". Over smoking a full packer brisket or even a 6lb pork butt is near impossible IMO. You can dry it out, you can not cook it 'done' enough, etc, but I don't see how you can over smoke anything.
That said, I will qualify this by saying I'm only using a
WSM. If you're keeping a low, even temp also the wood no matter how much you have in teh smoker is only going to smolder at that low temp for hours and not give off anymore smoke in 6hrs than it would in 20hrs. IMO, if you feel like you are oversmoking your meat what is probably happening is that you are cooking at way too high a temperature for too long.