This is a creative and interesting concept for defense, I think. At first glance I don't think it's illegal in its present form. Just keep in mind that the "bumpers" on the extensions must be included in the weight of the HOSTBOT, and cannot be thought of as the bumpers in the BUMPER specifications. Thus you can do pretty much whatever you want with them.
However, I think that this design is susceptible to easily being turned for two reasons:
1.) The extended frame creates more mechanical advantage for another robot to turn you.
2.) Even with traction wheels on the corners, there's very little weight on the outer corners in the extended positions; thus they provide little protection from another robot turning you.
If you're always beyond the Caution line and get turned into the opponent scoring zone by a high-traction high-torque robot, your robot
will receive a penalty, per
<G32>.
Thus, solve the anti-turning problem and I think that this is a solid strategy that blocks a good amount of the scoring zone.