Got it working!
Turns out if I specify anything other than the master station on the remote controller (pond-sprinkler), it works.
So, it looks like I just have to define a “dummy” station on the pond-sprinkler that controls no valve directly, but has the “Use Master” flag set.
The remote station is a master on the garage-sprinkler controller, so it will active the remote, dummy station on pond-sprinkler when any station (using master) on garage-sprinkler is on.
Still don’t understand the “configure as extender” thing, or why garage-sprinkler is on port 80 instead of 8080. But, I think I can get things working.