ChairGraveyard has done it! Go check out!

http://www.crypticcomet.com/forums/viewtopic.php?f=11&t=1297

It includes pre-made legions and praetors along with their art, but he shows how to add your own legions/praetors as well. It works with artefacts and relics too, but you won't be able to devise new mechanics, as such, but rather use existing ones. Choosing art... will be slightly harder. Hopefully, we'll build up a library of art for the mod and the community will be able to start modding SI!

It's still in testing stage, so any help would be appreciated, and even if you don't have time to help, take the time to have a look at it and see what you think!