Has anyone ever tried one of the bluetooth USB transmitters in there fit? Does it work? To BT music and calls.

 

I didn't even know that was an option!
I'd also love to hear anyone's experiences...

 

I don't see how this could possibly work, if you mean the kind of Bluetooth adapter that plugs into a PC. Honda would have needed to provide USB drivers and the appropriate "profiles" for Bluetooth to the audio system. Just because it has a USB plug, that doesn't mean it works everywhere there's a USB jack. Also, what would you use as a microphone?

There are quite a few Bluetooth audio adapters that connect to an Aux in jack, and still more that work over FM.

 

This worked in my 2009 Fit Sport: