I don't think it would be trivial (it's likely possible to some degree, but authentication and integrity checks might make it slightly more difficult), but the issue with this protocol is that you don't even need server control — any client with TextSecure installed can do this.

Note: I don't mean to disparage TextSecure by saying this. By all means, TextSecure is a kickass app and you should use it. I'm just trying to point out something that could be fixed in a future update.