A few things come to mind, first, plunge locks typically do have a tiny amount of play, it can be either side to side, or up and down, and as long as its minimal, its not a major issue, however, unfortunately (for you), I have seen plenty of mini socoms with no play at all, but, BEFORE you send it to MT, and spend the next year, or 2, or 3... wondering where it went, this may sound strange, but adding a decent amount of a good synthetic oil to the pivot area and thoroughly working it in can often cure a slight amount of play, hard to believe, but true.
Finally, I would be very leary of attempting to adjust the pivot screw, I tried to do just that on a MT i used to own, it was loktited in a major way, and all I did was strip the screw and I was being pretty careful. Frankly, Im not even sure that tightening the pivot screw will help since the handle slabs as far as I know cannot really move closer to each other, nor does the pivot have washers that i can see.