pfSense supports 40+ different DDNS services, and while I appreciate the purist view of MIAB’s REST-style API, it appears to me that most of the 40+ DDNS services take GET requests to update records.
I’ve posted to the pfsense forums to request support of PUT to update custom DDNS, so I’ll wait to see if that gets traction. I also found Dennis Murphy’s script to update MIAB DNS records from a cron task running on pfsense.