Starting with version 1.35.0, #Mitra will support POSTing signed activities to actor's outbox, as described in FEP-ae97: Client-side activity signing. Currently, outboxes are very limited: only Update(Person) activities are accepted. They don't produce any side effects and are simply relayed to followers.
This is just a demo and I'm not planning to develop a client. However, someone else could build it, and in that case I will dedicate some time to improving outboxes and adding other C2S related features.