O.K.
I am making real progress now using your method!

From a clean boot of my N900 using my Macbook as client:

I immediately initiate a PAN request from the Mac, it connects.
I then run your script.
I can ping 192.168.1.1 and successfully ssh into the N900 from the PAN.

BUT...

The phone isn't asking to start an internet connnection.
I'm going to start my gprs0 interface before initiating the PAN and see how that works...