Easiest way is to just set it in a mongrel.conf. Just create a file
ENV['site'] = "www.joesfancywidgets.com"
Name it mongrel.conf, and then start your mongrel with:
mongrel_rails start -e production -S mongrel.conf
The mongrel.conf file is loaded before rails or anything else and it is
a ruby script.
Try it out and let me know how it works.