g1smd - 8:58 pm on Jun 14, 2010 (gmt 0)
Your original plan was described as "redirect old site to new site", which then "redirects to script", which then "redirects to correct page". The rewrite eliminates one of those steps.
There is potentially another way; set up a proxy rule to pass the request arriving at the old site, silently and directly to the new site, and get the script on the new site to issue the redirect to the new URL.
Now there's just one redirect from old to new.
There's potentially other ways to do this, but it depends on what type of server software you have available.