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.
Please consider that some users have a certain last name in mind. Some of your users have partners who want to have the same last name. Thinking of a first name, then a last name, is not universal. Some users like to see the last names, then try to think of a good first name to go with one of them. This approach to name selection is now very difficult. Is it on purpose?