That diagram is how I used to have the toplink adjusted on my KK. It cut OK, but the travel allowed it to bottom out a lot and the runners would dig into the turf. Something that either bothers a person or doesn't. Maybe a lot more playing around with the height adjustment, the toplink and the rear wheel would have prevented that, or maybe adding check chains, which don't allow for any float downward.
Right now I have compromised somewhat and have the rear wheel is touching the ground supporting a little weight (don't know how it could support all the weight, not if it's hooked up to the lower lift arms), but the toplink is tight, supporting a good deal of the rearward weight of the brush hog. The cut is a little high when I hit a raised bump because I have no float for the bh to travel down, but I'm not leaving runner tracks or scalping either, which is what I'm after. I cut my field rather low for walking paths.
Not to beat a dead or dieing horse, but I still think the 3 pt is strong enough to support the weight by itself. It does in a box blade doesn't it?