Okay, so I coughed up my old 9850 and picked up a trusty used Treo 755p a couple of months ago (waiting out for the BB OS 10 device with physical keyboard). Then I recalled that I could once use the Treo to tether internet to a computer through a third-party program.
So, installed "USB Modem" on the Treo and the appropriate drivers on my MacBook Pro (running on the Mountain Lion OS). That got me internet connectivity on the Mac, albeit slow.
Then I created a WI-FI hotspot on my Mac, using a password to protect the link. Finally, I steered my Playbook to the Mac hotspot and viola - internet on my Playbook! It's as slow as Christmas, but it's free. Why? Sprint doesnt see my ancient Treo as a smartphone, and I'm not even paying for internet on the device - though it has access.
Will be on a trip in a couple of days and will be able to create a hotspot for both my machine and my wife's iPad - it'll be slow, but heck the price is right!