I would like to generate HTML Snapshots using Watir, hosted on Heroku.
Google's Full Specification for Making AJAX Applications Crawlable suggests using HTMLUnit... see How do I create an HTML snapshot? point #3.
HtmlUnit is a Java-only headless browser emulator; and unfortunately jRuby is not an option on Heroku. So HtmlUnit is ruled out (to my knowledge).
If you're interested I have another question open regarding HtmlUnit as a service hosted on Google App Engine... Making AJAX Applications Crawlable? How to build a simple web service on Google App Engine to produce HTML Snapshots? ... still waiting on a proven example/answer.
No. You need a full desktop environment to run watir. Heroku doesn't provide you with that.
You could use a service such as Amazon EC2