Why does the asset files being in XML matter? Are users expected to write their own tools or modify the files directly? Or were just commenting on them being nice files even though that point isn't important?
For static Meshes the workflow is quite easy - Unigine provide a tool that will convert many common formats including obj and collada. For animated assets you need one of the packages supported by plugins Max,Maya or SoftImage. Better support for Collada,FBX is on the todo list for this year however.
This is one of the places that will sink or swim a game company, especially a small one. If the content authors have to take weeks doing things that should ideally take days, that's a ton of time and money that the company loses.