Use Risugami's ModLoader and make it so that it's incompatible with Forge. People think that Forge is their lord and savior to modding so damn much that they don't even consider the original ModLoader, this is also good because most other mods use Forge, which is NOT compatible with the original ModLoader.
"If you've bought the Game, you may play around with it and modify it by adding modifications, tools, or plugins, which we will refer to collectively as "Mods." By "Mods," we mean something original that you or someone else created that doesn't contain a substantial part of our copyrightable code or content." Is what the EULA says. No substantial part. Meaning that if you run minecraft under it, it'll be fine.