Mongo has no support for upserting multiple documents in one query. The update operation for example can always only insert up to one new element. So this is not a flaw in the reactivemongo driver, there simply is no DB command to achieve the result you expect. Iterating over the documents you want to upsert is the right way to do it.
The manual on mongodb about upsert contains further informations:
http://docs.mongodb.org/manual/core/update/#update-operations-with-the-upsert-flag