Okay, for those who don't know how to make the FM transmitter output phone calls (note that the legality of it is probably along the lines of recording a phone call without permission in advance), which means that you probably didn't read the one thread that exists on the subject all the way through, if you ever bothered to look:

Qwerty12 made a rather simple hack (though it takes a few seconds to execute when you need to switch call to radio - but we have queen beecons by now) to make it happen.

Now go use that search function. I'm busy installing everything after flashing to PR1.3, and have a bunch of stuff I want to do with my N900 first, but I'll be happy to contribute towards further efforts to improve on it when I get around to it.