The rule I'd like to see (and, really, the rule I intend to follow) is "if you feel the need to write fanfic, write fanfic". This is true whether it's a piece of text or an Age. If I make up some D'ni history, I don't expect Cyan to pay attention or stay consistent with me. Same goes for a D'ni lamp, building, character, or piece of territory.
(I have, of course, committed Uru fanfic before. As recently as yesterday, in cooperation with Subterranean Restorations:
http://www.vortmax.org/subrest/index.php?page=project8 . I don't have any great plans right now for creating D'ni Ages... but I may continue inventing little snippets of D'ni culture, both on the forums and in Ages.)
EDIT-ADD: Sorry, didn't see that the new thread was up when I posted this. Replies there, please.