Is it practical to treat formwork so that surface concrete will not harden?
Yes, there are commercial products--glucose-base or other materials--which can be applied to the interior surfaces of forms that will kill the normal set of the concrete on the surface. The depth of the kill depends on the thickness of the coating, and some skill is required to obtain the precise effect desired. After the form has been stripped, the unhydrated cement can be brushed or waterblasted away to expose the surface aggregates.