These are interesting files that are found :
wnet/gdeliveryd/fightmatch.cpp = arena and others related pvp instances
wnet/gdeliveryd/kingdomwar.cpp = rift
wnet/gdeliveryd/battle_manager.cpp = frostgale fjord (12v12)
wnet/gdeliveryd/fastteambuild.cpp = PVE instances

It's just a guess but the key to editing instances requirement is likely in gdeliveryd.
It might be needed to compile a custom gdeliveryd to edit instances requirements.