Back at Pebble, we've been in a similar situation (installing via side loading or through an official "gallery" without a payment mechanism). For my game PinyWings we developed an external payment solution that later became KiezelPay – which ever since got extended to Fitbit and Garmin devices as well.
Currently, this approach would require internet access (not available on Playdate right now) but I can see how the concept could be changed to work in an offline scenario as well. Can set you up with the creator of KiezelPay if you like.
update: changed game link to black/white version for Pebble Classic to be closer to Playdate feeling – filtering for this, will mean that you cannot see this little gem though 