Seems it is even simpler than at first thought.
I emailed SBB to ask if in fact I could buy a Half-fare Card as a non-resident.
I now have a lengthy and very thorough email response which in summary says “yes you can”.
To quote from the response . . .
“If you’re a first time customer without an existing SBB customer number an online travelcard order is only possible, if during the automated process, the Webshop can verify your First and Last Name, as well as Date of Birth with a valid government issued photo ID and the respective countries have provided a verification process of some sorts.
Currently the following countries provide the technological means to do so: Austria, France, Germany, Italy, Liechtenstein, Switzerland (my emphasis in order to highlight ah ha moment)
If you have an ID or passport from a different country, the SwissPass Half Fare travelcard may be purchased only using these 2 options:
1) By post mail service: order form (including a copy of a valid government issued photo ID, as well a physical photo) and your handwritten original signature. It’s the same which is attached to this email.
2) Directly at any staffed SBB railway station ticket counter (payment in cash, Reka Checks or by credit card).
For legal reasons we require your original handwritten signature, thus can’t accept email orders or the likes.
If during the purchase/order, the postal address of the contract partner is not in one of the above mentioned countries, the automatic renewal is deactivated by default and can’t be overridden and no SwissPass card is issued.”
Their final recommendation was . . . .
“My advice, if you'd like to have a Half Fare travelcard on the SwissPass, buy it directly at one of our train station ticket counters. All it takes is about 3 minutes.
That way there is no paper work, you just order, sign the contract, pay and are good to go right away.
It also allows you to upload the photo comfortably digitally, have any other questions you might have answered and also set up SBB Mobile for an easy to use, convenient travel experience.”