So I took one for the team here, got (some of) the parts and tried it out...and it's a bust. The wiring harness *does* connect fine (at least physically), and I got the radar assembly connected to the harness. Unfortunately, when I started the car up, the SCC Reaction settings were not present under Settings > Drive Assistance (Per page 5-104 in the 2019 US manual). Resetting the system settings as whole and disconnecting/reconnecting the 12V battery to force-reboot things didn't work either. I'm basically using the presence/absence of that setting as a proxy for whether or not the equipment is detected and working.
Getting to the wiring harness requires that you remove the whole front bumper. Doing that was fairly easy once I got the confidence - after removing the various pegs and screws, you basically have to pull significantly harder than feels safe; if you've ever opened an old-style Mac mini with a putty knife, it's a lot like that.
https://static.nhtsa.gov/odi/tsbs/2020/MC-10172139-0001.pdf provides good instructions for how to get it removed, including demonstrating good grip positions.
Once the bumper is off, it's easy to get to the relevant wiring harness. My 2019 SEL (US model) has the mounting bolts for the radar unit, and the wiring harness itself appears identical to the stock one just with the addition of the plug for the radar unit. It's rather unfortunate that it doesn't appear to work, but I'll hold on to the parts if anybody *does* figure out what else is necessary.