Maybe it would be possible to talk to them about distributing a version that patches the mod like how there is a patch for the base game? That way one would still have to download there mod originally, and no redistribution of there work would happen.... just a patch for said work, you would have to go to them to get the original.
Sorry if suggesting this is too much, I am just thinking about how mods for mods work over in some of the other communities I am in, sorry.