The pivot point is determined by the balance point of the tube of the scope. Adding weight to the back end of the scope can move that point towards the mirror but you will find that the scope then will be heavier and not really give you any real advantage except a lower point of the side bearings which, in the case of a long scope, can make the eyepiece easier to access.
As to accessing the "hole" that all Dobs have at the zenith, the scope will still be just as unweildy to move about.
One detrimental thing with the side bearings closer to the primary is that the scope will be more sensitive to the weight of eyepieces and you will be building a method of balancing out the various weights of EPs and other such things as a camera sooner than with a balance near the center.
The large scopes usually don't have a solid tube which will reduce the weight at the EP end so they need to balance their weights nearer the primary end. I've experienced some of the scopes that you couldn't remove a large EP from without losing the point with the scope wanting to go vertical when the EP is removed and that is a pain!