Just playing the devil's advocate, as I use neither MP nor OMP (I prefer Symfonie), but... didn't OMP claim to be a "drop-in replacement for MP"? MP has the FM transmitter menu item in Now Playing...
Sure, mms://stream.polskieradio.pl/program3 for a start. But, I'm experiencing it with exactly 100% of streams I use (they work only if I mark the "video" button while adding them, and they won't resume after stopping, as explained earlier), so I'm quite surprised that it wasn't reproducible. Maybe it's just me and then, why? Could someone else could test it, too? /Estel