There does not appear to be means to specify that a referral is required for a patient to be seen in a service.  For example, a referral is (usually) needed for endocrinology service, while no referral is (usually) not needed for dermatology.

HealthcareService.serviceProvisionCode seems to be an appropriate element since ServiceProvisionConditions VS include 'free', 'disc', and 'cost'.

Is this appropriate?  Are there alternatives?

The new code (e.g., 'refreq' "referral required") could be used in implementation prior to publication since this is an example binding.

This would require a Terminology request, but it needs to be discussed and agreed to before a terminology request can be submitted.