Ben, I think you are going to run into an impossible challenge carrying save data over. There are so many things the save data holds in it including map data that will just plain break if you try to pull it over to a game that doesnt have identical map information.
Here is a possibility though...
If rathmor were say 20 maps, you could have map 0 - map 19 be rathmor, with map 19 being only a warp/save game map to transfer the file over to another game. In the sequel, you would have to begin on map 19 laid out identically to map 19 in rathmor, and then have maps 20+ be for the new game. Now as long as you dont go back to maps 0-18 your save game wont blow up due to changed event data, however your save will still have all the items attached to it from the old game, so you couldnt add new items, unless you did as holy described and stipped the player of everything but his important gear (maybe map 19 he falls into an abyss and loses everything but his kickass sword and shield?). Then you could delete out say 200 weapons/armor and make new ones for the sequel, you just need to make sure every item you bring over would be mapped identically in your item.prf and item.pr2 files between the games.
and last but not least, also holy mentioned, youd want to reset all counters to 0 on map 19 so you dont have any confusion.
So a reasonable scenario would be an event when you enter map 19, save, and then trigger a warp to map 20. In rathmor map 20 is nothing but an event that triggers upon entry to show you the ending (it doesnt save or do anything to your game after you warp). In the sequel, map 20 is the beginning of the game.
its worth testing...
