I understand that you are looking for more information regarding the log format, the transmission and that you are going ahead to write your own message multiplexer.
Looking into the man page of Linux Audit gave me the first hint about
The valid options for this are binary and string. Binary passes the data exactly as the audit event dispatcher gets it from the audit
daemon. The string option tells the dispatcher to completely change
the event into a string suitable for parsing with the audit parsing
library. The default value is string.
So it seems that it is possible to use a binary or string (palin text) for log messages.
Further research took me to the mailing list and AUDISP_PROTOCOL_VER2, as well to Understanding Audit Log Files.
Additionaly writing a basic audispd plugin and How to send audit logs with audisp-remote and receive them with netcat will help you too.
I am aware that this might not fully answer your question, but it should give enough input to start.