Have an engineer look over your 1/2" plate idea. The lift manufacturer might help out. I'd be concerned 1/2" might not be stiff enough to add much effective footprint.
My plan is to remove a section of concrete as wide as the whole lift and front to back a few feet more than the bases. Dig out under it for a well overkilled reinforced concrete slab/block dovetailed under the remaining slab. This would be the cleanest solution and is recommended by the lift manufactures.
