If anyone wishes to cite the rules in cases like this.. T654 section 8 (general halbtax issuing rules) and section 91 (1-year halbtax). Nothing here makes any mention of residency requirements. All you need is an address to send it to and some form of ID (remember, if you're asked for an "ausweis" they don't necessarily mean an Ausländerausweis - a passport might well be enough. It was when I got my first halbtax.)

For some bedtime reading:
http://extranet.ch-direct.org/dcs/users/71/T654_d.pdf

An address in Switzerland will most likely make things simpler for them to send you the card. When my other half bought her halbtax at Zürich HB travel centre the clerk did a bit of tooth-sucking and asked if she had a Swiss address they could use. I've even heard rumours of the plastic card getting sent to the station and the buyer getting an email asking them to come and collect it - although it might have happened, it'd still very much count as "above and beyond the call of duty" for the staff. I'd guess.

The Halbtax with Visa card is obviously a different kettle of fish - you're applying for a credit card if you apply for one of those, so expect to have to produce various proofs of residency.