The Z-bot power drawbar works exactly like this See http://littlemachineshop.com/products/product_view.php?ProductID=3034&category=209730719
. Brian has one running at the CNC workshop last summer, and we had it running at Cabin Fever in January..
I talked to the guy that developed it and he told me about 600-800 pounds was enough for the Tormach tools.
I built a similar, but simpler one on my Jet knee mill. I have my table move for the z axis, so my quill is free to operate by hand. I put the bellville washers under the drawbar, then I added a simple metal bridge above the top of the drawbar. To change a tool I simply drive the quill up with the hand feed until the top of the drawbar hits the bridge, then push a bit further to compress the springs and pop out the collet. It works fine, but I am a little light on the bellvilles, so I did have a collet slip once.