How to use Bluetooth PAN and WiFi simultaneously in OSX On my Macbook Pro I connect to the Internet via Bluetooth on my iPhone, which does not work when connected to a WiFi network.

How do I tell OSX to route WAN traffic over my Bluetooth connection, while still being able to communicate with other machines over WiFi?
You should be able to achieve this by making adjustments to your routing table. Have a look at the manual page for route on how to tell your Mac to send your default traffic over the PAN, and traffic for those other machines to your wifi interface. 

One of your challenges I know from experience is going to be that your Mac will drop them upon reboot. And when you address this and make the routes persistent, the second challenge occurs in that and the routes will get in the way in situations other the one for which you need them.