@Gargron I think it's an important feature since this is an anti-censorship tool. One solution (off the top of my head) would be to enforce nodes to send users read receipts with signed hashes of their message as it's posted. That is proof that the node received each particular message. If a node is suspected of censorship the censored user can verify with timestamped+signed receipt from node. If the node doesn't send you a receipt in the first place you can assume it won't relay your message.