Well, Splatt, lets do a little critical thinking here, and maybe you can answer your own question.
First of all, most of the layout blinds I have been in are big enough for me, a gear bag, and my gun. Second of all, the level of training required to keep a dog laying down in the blind with you while you "pop" up for the shot, would be damn near ridiculous...and I'm not talking about not breaking, I'm talking about the dog staying in the down position. Thirdly, with the dog in the blind with you, you will have to pick up all of your birds via "blinds" because the dog won't see ANY of them fall. I don't know about you, but I have enough to worry about in the blind than my dog, and my dogs safety. Your working calls, your watching the birds, your making sure your gun is in the right position for mounting...you are not paying attention to the dog. Sit up to shoot, and the one time fido moves, as your bringing the gun up...well, nothing good can come of it. If you haven't figured it out by now, spend the 100 bucks for a mutt hutt...it will save you in the long run.
"Fundamentals are a crutch for the talentless" - Kenny Powers