Upsert allows constructing a resource path and providing a desired after-image. If the request is executed successfully, the identified entity will exist and look somewhat similar to the desired after-image.

Unfortunately this does not combine with optimistic concurrency.

If an operation on an entity requires an ETag, then services MUST refuse requests without an If-Match header.

If the client doesn't know (or care) whether the entity already exists, it cannot produce a valid If-Match header, because even the wildcard * does not work: HTTP defines that if the field-value is "*", the condition is false if the origin server does not have a current representation for the target resource, i.e. it is false if the entity does not yet exist.

I don't see a way to make upsert work in combination with ETags.