Am I correct in thinking that for the latest beta you have

1. Corrected the target window of incoming echo-messages
2. Prevented these messages from triggering text events

This is pretty good, but means that the message will be left in znc's playback buffer when "Auto Clear Chan Buffer" or "Auto Clear Query Buffer" znc flags have been disabled. These require a "clearbuffer" message be sent; without an event to capture these messages to send the clearbuffer, all sent messages are replayed on every reconnection. PARSELINE can probably be used to handle these incoming messages, but that seems wasteful.

Disabling the CAP from the CONNECT event was too late and the playback buffer was still sent, but capturing the RAW event was soon enough to prevent it. So barring any other complications, this is probably what I'll be going with.
Code
raw CAP:*LS*znc.in/self-message*:{ .raw CAP REQ :-echo-message }