At this point in time SiteMinder does not offer iCal, nor offer any free API availability solutions.
SiteMinder are mainly focused on leisure tourism for hotels and do not do much business with digital nomads.
Unlike most channel managers SiteMinder also charge large fees for OTAs like us to send bookings.
SiteMinder have repeated rejected our applications over many years so we expect it will be some time their model changes and SiteMinder clients can integrate with Nomad Stays.