@h1uke Thanks for helping.
For the record, just to report how got it to work... though I can't say it has been easy.
1- I never found a way to send the multicast from a specific host interface, I disabled the software interfaces of my virtual machines instead. Then the python multicast programs in my first post worked fine.
2- I had to punch a hole in the Kindle firewall to accept UDP on port 10000.
3- Since my Kindle connection is via the wifi interface only, and Launchpad will only join the multicast group after receiving the "start RCE" string on port 10000. The router will not establish a multicast group without yet.
4- I had to connect my host via wifi, and I had to start listening to that same multicast group from my host (using the multicast_receiver python program).
5- By IGMP magic, this caused the router to add the multicast group to its wifi interface, enabling routing.
6- Only then, upon receiving the multicasted "start RCE" would Launchpad accept to pass further text strings as shell commands.
Definitely easier when the Kindle is connected to the host via USB networking (and no wifi routing is necessary).
Fun learning, but unless I had an army of Kindles to command I would stick to SSH unicasts.