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I assume you only want to do this from an admin side? I don't know if anyone has done it, and someone else would need to comment on security, but it certainly is possible.
You could use 1 script which let's you browse your XOOPS folders structure. Another script which could take a selected file and display in a textarea html field. Last you would want a script or function which overwrites the file.
It isn't a massive challenge to write these scripts in php. I wonder what the core team thinks of this. A module could be built that basically has just and admin side and the described scripts above.
Personally I would never do this simply from a version control standpoint, but I do see how it might be useful to some people.