My moss wall I made, due to gaps, allowed fish to get behind it but the slim space made it hard for them to get out, so I imagine they would get behind the 3D wall too. (They completely freak out because they get "trapped" and nearly die of stress.) Depending on the design you could possibly figure something out.
I have a 3D background for my 72G to 75G I traded for and haven't had time to get all my tank tubes and wires and plants moved to get it in yet. Then there is the possible issue of mine not staying in place unless I caulk it. Move out fish is possible issue for you.
Hope you would be able to have a dry tank to do the build.