CodeableConcept and nullflavor OTH, UNK

We're going to use FHIR medication resources for our national list of prescriptions (by decision from the eHealth Agency). We are also going to use SNOMED CT to code reasons for prescription, but since this SNOMED value set is never going to be complete we need to represent something like nullflavor=OTH. The specification generally does not include nullflavors, although there are extensions like DataAbsentReason. According to this Zulip thread there has been a design decision to not use a nullflavor like in v3. https://chat.fhir.org/#narrow/stream/4-implementers/subject/DataAbsentReason.20extension.20or.20ValueSet.20with.20UNK.2FOTH.3F

How do others solve this problem?