That could be build into deltachat:
Support for creating “relay groups” in deltachat, and therefore becoming an admin of them.
In such groups the admin would send out messages to the group using BCC, and all members can technically only respond to the admin(s). But if this group is a full relay group, instead of just a news feed, the admins’ deltachat instance(s) could automatically BCC-forward the replies and messages received from a valid member, to all members.
If the deltachat instances of the members could deduplicate the messages locally, the relay group could even support multiple admins and relay-instances (for redundancy and fallback).