I'm wondering if Public Mobile might be able to offer something similar to KnowRoaming? Basically they have deals with other providers to allow whatsapp messages and calls on their networks for free. KnowRoaming doesn't charge for any of that data usage.
If they can do it, I don't see how Public Mobile can't do something similar. Of course in order to do it, PM would likely have to have some sort of paid roaming plans with the providers for all non whatsapp traffic.
Anyway, just an idea!