Hey Iestyn
The CSV Data Source asset is not supported as a specific asset type within the EES interface. You should still be able to redirect and load it from the asset finder with it's own webpath/_edit though. Which will let you edit some basics like details, metadata, linking et al. The only way that you 'might' be able to expose the upload feature to your users might be via the EES plugin system and something like > after details screen loads > try adding a new section to the details screen > use simple edit keyword replacement to load in that section from standard simple edit > do some code to clean up/allow user to upload. All that would have to be done in a very 'safe' way.
That's not really trivial work. It's custom stuff. But can be done is more of a controlled/supportable way via the EES plugin approach. That way it can be turned off (commented out) during upgrades. And then reviewed post upgrade for any issues. And requires intermediate to advances JS skills as well as knowledge/skills around the Matrix simple edit system. Specifically how you can use simple edit keyword replacements to load specific screens/sections.
Might be helpful to engage with the local Squiz office for assistance to avoid anything that might cause pain/grief in the future.
Cheers
Scotty