No, it isn't only for the original demo. Follow ShellShock's first link. On the last page a utility that extracts data.pack and loads custom battles is described. It works. I'm not sure about editing. The format followed in the descr_battle.txts is simple, and it's easy to change the units and factions involved in a battle, but changing their starting positions can't be sensibly done without some other tool. Somebody else probably has more information.