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.
Use Speedify.
(I don't work for them or have financial interest in them. I'm a user.)
What this software does is to combine multiple internet connections.
I am combining a bluetooth PAN and Wifi.
Blue tooth Pan from a Sony Z3 Xperia Compact phone on EE in the UK and Wifi hotspot from another same model phone on Giff Gaff (a MVNO running on O2).
I'm doing this to get better coverage on the train between London Waterloo and the south coast of England. All operators have their weaknesses (areas of no reception), but I have found that the weaknesses occur in different places, so combining 2 networks improves the overall internet connection to an extent.
