sikboater, Since nobody else has come back to you on geting rid of the brush pile, I would like to offer a couple of suggestions - neither of which might suit you of course!!
On one place I owned and where it was necessary to get rid of the scrub in a hurry, we deep trenched and pulled the next strip into the trench, working our way across maybe 8 acres. It was then successfully grassed down to pasture.
The other possibility - and this involves quite a bit of work, is to clear out the thickest stuff (3 or 4 inches I think you said earlier) possibly using it for firewood. It could be chipped of course, but may cost too much. If there is only small stuff left, then treat it as a huge compost heap. It will eventually break down into good compost. How long depends on your weather. If it dries out in the summer then it will take a lot longer than if it is kept moist. If you are in a position to spread animal manure over the heap, or green vegetable matter, then the time will also be speeded up. It all has to do with the combination of the high amount of carbon you have in there in relation to a low amount of nitrogen. Air and moisture are the only other limiting factors.