My apologies for tagging it as a bug.

No worries, I understand where you are coming from. I would think though, that your use case is quite uncommon. Unless you are using the constraints for some developer centric validation itself, I am not sure why you would have these type of expressions in your message.

I think my own custom MessageInterpolator here will be overkill so I'll just escape the string for now

That's what I would suggest. But as said, the best bet you have in case you need it, is a custom message interpolator. You could always start with the one provided by Validator.