Would it really be worth it to have something custom built for this?

FM Modulators are readily avaliable, easy to install, and relatively cheap...

or if you're serious about quality...

You can replace the RF deck with an aftermarket MP3 deck with an AUX input for probably less. I was concerned about losing my 6 disc changer at first, because it is really nice to have, but after installing a MP3 deck and Sirius tuner, I can tell there's not going to be a problem with missing the stock changer. Just dropping in the deck made a big difference, and then switching out the speakers made even more of a difference.