It is possible to design a mounting board which is dielectric and temperature resistant as well instead of out of metal as the current board is. The proposed material would be 1/4" thick mica plate with standoffs attached and holes as needed for complete interchangeability with the current design.
The cost would be slightly higher than the metal bracket but there may be some advantages with regards to thermal overload or possibility of electrical shorting to the mounting board.
Is this something that would be of interest? It could be an improvement over the current design and add an additional safety factor to the design.
There is no reason why you couldn't do this. You will need to run ground wires from the ground bus to the OpenEVSE controller. Neither material would affect the internal temperature or safety. There could be increased electromagnetic interference or radiation by removing the ground plane.