Play music from phone on tablet through bluetooth?

Not sure about bluetooth, but I think that you can stream music between Samsung devices connected to the same hotspot. Its in the feature list for Kies Air, haven't tried it though. I do see the music icon when when I access my phone via my tablet though.

 

If anything, you'd be able to stream the music directly from the tablet to your car's Bluetooth audio. I'm nearly positive you can't stream media between two android devices through Bluetooth.

You may be able to do it through USB as long as the tablet supports USB hosting (I think the nexus 7 does).

Is this what you are thinking:
Custom Nexus 7 in-dash installation makes me wish I had cutting skills [VIDEO]

 

I was looking this out of interest myself. And I wasn't happy with what I found.

How to stream audio from PC to Android Device over Bluetooth - Android Enthusiasts

In a nutshell, its not possible (or at least your going to have to put in more effort than its worth) to stream Bluetooth audio between two mobile devices. The drivers only declare they are BT Audio sources, rather than a "sink" (receiver). Technically I don't think there is an issue at the hardware level, but you would need to rewrite the drivers to allow for this and add that functionality to Android itself if it doesn't support it.

While you have a noble and great idea, I think getting that much would be a huge feat in and of itself.

Let's put it this way, unless your an Android expert, it would probably be easier swapping out the radios (and I'm sure you don't want to even do that).