What's wrong with a lame map editor? Custom maps are arguably the best form of fan-made content. Documentation, Tutorials and consumer-friendly map tools don't grow on trees.
You also don't require the source in order to make modifications. Not if you want to implement extra abilities but if you're out to tweak the data values you're home free (given that you can read the data like the formats you mentioned earlier).
I believe Halo had some extra tools released, that fits your precedent description. Not that the inclusion of tools with TFU will depend on such precedents.
-"The glass is neither half-full nor half-empty: it's twice as big as it needs to be."