Any code reference to mounting a panel on concrete block vs plywood?
No problem.
Check with your AHJ… a town in my area requires the panel be attached to “treated plywood/lumber” prior to mounting to block/stone/brick/concrete.
Just remember a few things…if this was a Damp or Wet Location ( and I am not saying it is…I’m just saying ) then you would need to be aware of 312.2 as well which would require the 1/4" airspace.
Otherwise, just check your local AHJ for any localized mounting surface requirements.