Trepans approach should be good for this. This would work with a lot of mods and probably all maps.It should be really minimalistic IMHO:
The other thing is to provide inexperienced users (os and/or spring) with an easy and consistent way to (un-)install all kind off addons for spring. I am sure this would be very helpfull for new users, but probably means a lot of work.
It don't think to say: "If a mod is released as .zip, then it's the modders problem" is a god way. Any serious mod/ai should contain at least a readme and a changelog. So you need to distribute at least three files. A consistent way to get access to this files would be very helpfull for normal users.
Unfortunate I have no idea how to write a crossplatform gui-application (apart of java)