Can I create a URL that opens up a text message ready for a user to send?
Yes, try this link on your mobile phone:
However, unfortunately it appears at the moment, that it is not possible to populate the body of the text message. See http://www.nfc-research.at/fileadmin/papers/2011/smsuribug_mr.pdf for more information.
You may find http://beradrian.wordpress.com/2010/01/15/special-links/ a useful source of URL commands that can prompt actions on smartphones.