Bruce,
I am having a wonderful time following your progress on this. I am very excited to see someone putting this much effort info reverse engineering and re-implementing these systems. This is how we move forward in technology and amateur radio - by breaking the mold and sometimes re-inventing the wheel.
A note I have about something you said...
According to app_rpt documentation, this feature is actually supposed to exist already, in the form of the K key in IAX text.
Sadly, it appears to never have actually been implemented. I brought this up as a feature request in another topic recently, and a Github issue was opened to hopefully look into this more at some point.
Nonetheless, excited to see some talk and development in this area! This is something that app_rpt has been desperately needing for a very long time.