Douglas, if your interlocutor is a really consistent Bayesian and has a probability estimate of exactly zero for whatever-it-is then I advise you to talk to someone else instead. If (as is more commonly the case) their prior is merely very small, then what you need to do is to present them with evidence that brings their posterior probability high enough for them to think it worthy of further discussion.
This is in fact exactly the problem you face when discussing anything with anyone (Bayesian or not) who finds your position or some part of it wildly improbable. At least a Bayesian is (in principle) committed to taking appropriate note of evidence.
Constant, I think even the strictest subjective Bayesian should be happy to agree that in the presence of suitable symmetries the only sensible prior may be determined by those symmetries, and that in such cases you can save some mental effort by just talking about the symmetries. Just as you can talk about the axioms of number theory or logic or whatever even if you think they're really empirical generalizations rather than descriptions of Platonic Mathematical Reality, and usually when doing mathematics it's appropriate to do so.
Followup to: Illusion of Transparency: Why No One Understands You, Expecting Short Inferential Distances
A few years ago, an eminent scientist once told me how he'd written an explanation of his field aimed at a much lower technical level than usual. He had thought it would be useful to academics outside the field, or even reporters. This ended up being one of his most popular papers within his field, cited more often than anything else he'd written.
The lesson was not that his fellow scientists were stupid, but that we tend to enormously underestimate the effort required to properly explain things.
He told me this, because I'd just told him about my experience publishing "An Intuitive Explanation of Bayesian Reasoning". This is still one of my most popular, most blogged, and most appreciated works today. I regularly get fan mail from formerly confused undergraduates taking statistics classes, and journalists, and professors from outside fields. In short, I successfully hit the audience the eminent scientist had thought he was aiming for.
I'd thought I was aiming for elementary school.
Today, when I look back at the Intuitive Explanation, it seems pretty silly as an attempt on grade school:
(Then again, I get a roughly equal number of complaints that the Intuitive Explanation is too long and drawn-out, as that it is too short. The current version does seem to be "just right" for a fair number of people.)
Explainers shoot way, way higher than they think they're aiming, thanks to the illusion of transparency and self-anchoring. We miss the mark by several major grades of expertise. Aiming for outside academics gets you an article that will be popular among specialists in your field. Aiming at grade school (admittedly, naively so) will hit undergraduates. This is not because your audience is more stupid than you think, but because your words are far less helpful than you think. You're way way overshooting the target. Aim several major gradations lower, and you may hit your mark.
PS: I know and do confess that I need to work on taking my own advice.
Addendum: With his gracious permission: The eminent scientist was Ralph Merkle.