Thanks, everyone! Here's what I managed to do:
0) Start the managed Jacktrip Server, and note its IP address.
2) Run the JackTrip app on the Mac.
3) Configure JackTrip's RtAudio back end to set its input device to a microphone that's fully turned down (so I'll only be heard once on the remote mix, from my JackTrip Raspberry Pi's input), and for its output device, choose the BlackHole 2ch driver.
4) Connect to the managed JackTrip server using the IP address from step 0. (Note: I was unable to successfully stay connected to the remote server while I had the Jack audio drivers installed on my Mac -- I had to remove them and just use the RtAudio back end.)
6) For Audacity's Microphone input, choose the Blackhole 2ch input. (Its input end is already connected to the JackTrip server's input from step 3.) For its output, put on a set of headphones so you can hear what's going on.
7) Set Audacity's Project sampling rate to 48000 Hz. That appears to not be a requirement, but it does match with what JackTrip is doing, so it seems like a good idea.
8) Press record in Audacity. If all goes well, you'll see the JackTrip output appearing in Audacity.
I've still got a couple of mysteries here, though:
1) Having to remove the Jack audio driver in order to get a stable connection with the JackTrip app - that just seems crazy, and says I must have had something wrong, but I wasn't able to figure it out.
2) The level of the recording volume input from the JackTrip server is pretty low when it's just me connecting in. I haven't tried recording the whole group yet, so maybe that will take care of itself?
Any thoughts on this setup? It seems simple enough?