[RRLedford]

Some thoughts:
Forget the heavy bars, and just have dangling loops of 1000 lb kevlar cable hanging from near bottom center of first elevated robot, so weight pulled there won't tilt it too much
The first robot hanging ideally should drop these cables loops to hang at one or two (low/high) agreed upon heights from its undercarriage, so that they can then easily be engaged by next bot just driving under them with a static hook mounted at the pre-agreed height. Then if 1st bot up tower still has enough remaining power to pull both itself & tow the other bots further up (at 250-400 lb force) add 3 or 6 more points. Also, if bot on floor can pull with its its hook too, even better. The key is locating these pull points as near to vertical axis of center of mass as possible to avoid excess tilting.

-RRLedford

Note: The bots hooking on may cause the elevated robot to dip below height needed to score 2.