It would be instructive to identify the specifics of what led you to want to edit an .ndf directly rather than use the UI. It could help the product team evolve the UI to make the activity more efficient.
For example, in FLOW services, mapping is a tedious set of clicks and drags. For documents with more than a few fields, it is downright carpal-tunnel inducing. 
It would be nice to be able to quickly type or cut/paste field names to be mapped rather than clicking/dragging.
Perhaps you can describe the details of what you’re doing and how the model editor inhibits or makes it harder than desired.