More to the point:
The idea of gunboat is simple: a game with the same mechanics as Diplomacy, but without the ability to negotiate apart from submitting orders. Gunboat is Diplomacy with no vocabulary. A game in which the draw, cancel, pause, etc. buttons can be used to communicate and negotiate is a different game -- it's standard Diplomacy with a limited vocabulary, but not no vocabulary.
Further, having any sort of canonical meanings to these buttons, in my view, is not a gray area, but clear cheating as meta-gaming. It's agreeing with another player in the game to use a set of pre-arranged signals in order to negotiate and communicate.
The following would be equivalent. Let's say that I talk with you ahead of time, and we make an agreement that anytime the two of us play a public press game, we will each send secret messages hidden as every tenth letter of our public press. So, in the event that we find each other in the same game, we'll recognize that and be able to communicate privately in addition to the public press that everybody else gets. Or, if you prefer, can a group of people pick a shared PGP key and post encrypted public press, hoping that somebody is able to read it and responds accordingly? Would that be legal? I mean, it's all public information, isn't it? Well, no, it's not, because there's a private key.
And this isn't a gray area. From the rules:
3. Don't work around game messaging/press rules.
If you enter a game with limited press/messaging (such as global-only) then you can't use other methods such as email or PM to bypass this.
I don't see how using these buttons to communicate can be seen as anything other than working around the no messaging rules of a gunboat game.