How to load assets that aren't part of a build

We want to allow our players to play our game on their own maps. Esentially loading assets that aren’t part of a build

There is the manifest_UFSFiles.txt that is easy to work with, but the assetregistry.bin associated doesn’t seems to be designed to be accessed or modified manually.

Overall the question I have is; how can I add and load assets that are not part of a build?

I think Unreal Tournament will need this, how are you guys going to handle that? Automatic downloads etc?