Well if I say: "I will build a hot-air balloon" then it's reasonable to interpret that as agreeing to the rule "I have to build a hot-air balloon", so if I don't, I'm cheating.

That doesn't seem reasonable to me, actually. I interpret it as 'I intend to build a hot-air balloon', which is much weaker evidence about future world-states even if it's true. (It's also stronger evidence about current world-states.)

The default position is that speech is true.

This strikes me as naive. In my experience, most people don't lie without a reason to do so, but also most people will lie when they do have such a reason, and such reasons are fairly common. Our society is built on that assumption, in some ways, even - it's practically required that one make up an excuse to leave a conversation with an annoying person rather than tell them that you don't want to talk to them, for example.