I'm not quite sure what benefit you're expecting from communication via the Bitcoin blockchain. The way I see it we have enough problems with bloat as is, as such using the Bitcoin blockchain for storing communication data beyond transactional information seems rather wasteful.

Thing is, "the only requirement being they are using the same software" isn't quite true. There's another requirement, and that is having the money to send a transaction (or a message, as would be the case).

I might not quite understand what line of thinking you are going for though.