Square is doing a test run in a small set of Square Cash users.

Currently, with the Bitcoin transaction times right now at 300 minutes (and a peak of 8,000 minutes 2 days ago), and the high cost of transaction fees right now around $10 (and a peak of $35 a couple days ago) It isn't viable for a POS.  There are lots of other newer cryptocurrencies that have learned lessons from bitcoin and have fast transaction times and minimal fees.