So I signed up at identi.ca a while ago, but never really did anything with it. I also found Yahoo Pipes a while ago, and put it on my Todo List to look at, but that was about it. I don't know why I didn't think to look at it to do this.
Anyway. Pipes lets you manipulate pretty much anything, RSS feeds being one of them. You can feed things around, or pipe them, and alter contents, do whatever.
After a little Google work, I found this pipe that converted a users Twitter RSS feed to something twitterfeed can use. The Twitter RSS feed items are normally prefixed by the username, so for twitterfeed to use it, it really needs that stripped out, and the pipe does it. Check it out by just popping in your Twitter username and hitting "Run Pipe".
So now I use twitterfeed to push my Twitter stuff to identi.ca. Win.