Is there a way to publicly view an email as a webpage?

One option, if you don't care about tracked links and mkt_tok, you can use the view as webpage option on a sample send.  Just make sure your email is fully approved before you send the sample.  

The link to view the message in a browser doesn't give me a unique url.

I'm afraid it does it give you a unique URL, but that's exactly why you should never post it publicly, as it's linked to the original email recipient.

There's no way built into Marketo to accomplish this. At least one service exists, if you search past posts.