The problem is that you need something to store the dynamic schemas in. That is where the Schema Registry comes in as it provides a UI and api to add/update/delete schemas. These can then be refrenced from NiFi.
It looks like AvroSchemaRegistry allows you to do the similar, minus the ui/api. So you would need to create your schema in your flow, as attribute, and send that to AvroRecorderReader configured against AvroSchemaRegistry. You could use some other data store to store these schemas, but you would need to pull them out into an attribute of the same name configured in the Reader and Registry.
The latter method does not give you a way to manage all the schemas, which is why I reference the Hortonworks Schema Registry which does include ability to manage, version actual schemas.