Does anyone know if access floors can be installed over plywood substrate, above grade, and what the deflection criteria would be?
No reason it cannot be. Talk to your structural engineer.
The loads are concentrated at the posts so the plywood flooring might need to be thicker or thickened.
Access floor systems that provide lateral stability by cantilevering off of a concrete slab will likely not be appropriate. Thus you may be limited in the system you use.
The deflection criteria would be the same as for the type of use that it will support.
Check on fire safety. Since the wood floor is combustible, the cavity between structural floor and raised floor may need to be protected. For example, if the room above is sprinklered, you may need sprinklers in the cavity as well.