What if for user created dungeons, we allow the character who made the dungeon to supply his own loot from his character's inventory (or chest inventory).
And have a rumors (partially factual list) of potential loots available in created dungeon to entice exploring the dungeon.
This could keep the loot system from being abused by only having standard(player-found and looted) treasures in user-created dungeons.
The incentive to make dungeons could be called DM favor and give whatever good bonuses the devs see fit. Just an idea.
Building on that idea if there is a super popular user created dungeon: have the DM hire WF artificers to create loot for his dungeons(like Santa's elves) by using a essence stockpile and mat stockpile supplied by the aspiring DM. I know it sounds a bit goofy, but by doing a user crafted loot supply, then the DDO economy perhaps could be saved in the event of user created dungeons.