We already have Little Joe + Big Joe is on OH and they work.
If someone wants to work on Mercury, count me in.
I've already retextured the capsule, the Atlas/Redstone, done some VC graphics, etc. Don't see any problem in reusing the existing meshes.
Don't know what's on the docs about redistribution, but I think that it might be allowed.
Now, to simulate systems, we need custom code or a way to get LUA to accept VC clicks (to have a working VC).
Nevertheless, it's possible to have a rough implementation using SC3 + LUA for working instruments + Dx9 client for the periscope (using reflections).
Don't' see how UMMU could be used. Mercury had no docking port, and no EVA possibility.