I also have my fuel cell in the trunk, which does require a sheet metal firewall (min. .032 steel or .040 aluminum) for NHRA tech. Not sure about only a trunk-mounted battery.
That makes sense with the fuel cell. A quick Google search came up with this quote supposedly from the NHRA rule book:
"Any battery MUST BE mounted in a SEALED NHRA APPROVED battery box IF the car does not have a SEALED FIREWALL from the driver compartment to the trunk"
So the sealed battery box has me covered in terms of the rule. It doesn't mean that a firewall isn't a good idea though.