Use HTTP GET or POST to prefill
For example, a link such as
http://www.secondlife.com/join/?u=b494b5309a2a0ef3a6ea71c8bd7e4bd2&last_name=Galaxy
should take you to the registration page and have the last name "Galaxy" pre-selected in the drop down (assuming it is in the pool of available names.)
I have this nice web page that catalogs all of the available names, http://slnamewatch.com/botlog/last_avail_log.html , but several of my users have expressed disappointment that the links do not drop you directly onto a pre-filled registration page. Unfortunately I have to say I don't see how it would be possible to prefill. Please add a feature to the join now page that takes parameters and pre-fills the form.