I don't have a direct answer for you. But personally, I can't see why its not possible. You can look at it as a aluminum frame with PVC support instead of a PVC frame with aluminum support. The PVC can distribute the vibration as well as give a basic stable platform. Add in the aluminum supports at the right places along the load path and it strengthens the system by 500% enabling an increase of forces.
I don't thing anyone has actually created a benchmark for the 7x7 or 12x12 to see just how much it will tolerate. It has a safe rating right now, but add in the right bit, spindle, perhaps replace the axis motors, and press the feed-rate all the way up while carving into some soft aluminum. Increasing the material and resetting the feed-rate until a point on the frame shows damage. I'm sure it will be along the load path somewhere, but I'm very curious about just how tough the PVC frame is.