Very interesting idea. I probably won't have enough time for a 7DRL, but would happily develop some AI code. In the future, we could make another jam, an AI competition. I would be very interested in organizing such a thing.
About the architecture, I think doing plain text communication might be a lot of work, because you have serialize the game state, which can be pretty complex in a RL. And then every AI developer would have to work on parsing it. Instead, why not just enforce a language, like Lua or Python? AI code is usually fairly compact anyway, so I doubt anyone would mind.