I don’t know if it uses any assets from Fallout 2, but if so, the Tale of Two Worlds mod makes use of Fallout 3 and Fallout: New Vegas assets, and Bethesda’s obviously okay with that. It, IIRC, requires that both Fallout 3 and Fallout: New Vegas be installed, so I imagine that that route would always be available, even if Bethesda isn’t okay with this as it stands.

You can always legally distribute a set of non-derived-work changes to a copyrighted work. End users can apply your changes to their own copy of a copyrighted work to create a derived work from a copyrighted work. This is true even if it’s not legal for them to then subsequently redistribute the derived work – copyright law doesn’t prohibit creation of a derived work, just distribution of same.