You wouldn't believe how often I've attempted to make myself available to other areas of the company that have needed help. But I was hired to write code, so I should just focus on that. ;-)
If in order to succeed you need to clean the bathroom (figuratively speaking), just go do it. Go beyond the job description; it's a startup, after all. Job descriptions are worthless, until you have a decent size and need some bureaucracy to survive.
If the founder is not seriously expecting you to do go beyond your comfort zone, you should seriously question his ability to lead the company to the next level.
"Hey, the company would be a lot better off if I spent some time on X, instead of my usual responsibility Y."
But as I said, if I were ever in this position, I'd just pack and go. It's hard to build a successful startup; even harder if the CEO thinks he/she has all the answers and is not open to collaboration and sharing the burden of the key early decisions.
This is particularly true when the company is small; not so much when you get to triple-digit headcounts.
Like airbnb:
But seriously Loren I think that the company that you are at now might have already marked you in a way that could limit what responsibility and opportunity they give you. Because they know you're not happy perhaps (and especially after reading your comments if they do). Personally I think you are cut out more for your own startup if you can in fact "wear many hats" than in one area like engineering.