It sounds crazy enough to work....maybe.
What about ice movement? Concrete has tremendous compressive strength, but terrible shear strength. In essence, the only lateral strength will be that of the PVC pipe. A few rebar verts in the pipe will help, but a true concrete pier would have a combination of verts and horizontal ties. An 8" dia is nearly impossible to install a rebar cage of verts and ties. You can't make a tie small enough and the 8" dia is too small a mass. So, we return to the PVC pipe to provide lateral shear strength.
If there will be little lateral force, it should work. The next issue would be how many to support the proposed load.
