At present, the only way to get PricingPlan data for a user is for the current user. You can't lookup for any other arbitrary user.
That's no good to me, as I'm trying to write an HTTP function to allow an external system to query the PricingPlan data for an arbitrary user.
Unless I'm missing something, this isn't possible. It means I'm going to have to write my own Pricing Plan functionality into my own database, which is a shame seeing as most of the functionality is already there!