BillyS - 2:13 am on Apr 20, 2013 (gmt 0)
I suspect you're a mind reader or you're familiar with Joomla 2.5 :)
After a bit of a cry, I came up with the following, which seems to be of the same method you proposed.
Redirect 301 /rss.feed http://www.example.com/index.php?format=feed&type=rss
Which doesn't make the new URI look pretty, but it does provide a working URI to those looking at the old one.
I'm going to test your suggestion too.
RewriteRule ^rss\.feed$ /index2.php?format=feed&type=rss/ [R=301,L]
If they both work, I'm hoping Lucy will weigh in with a best practice. :)