Alright, with some encouragement from gpez, I tested it out today, and everything worked as I had hoped and as gpez suspected. To summarize:
What I was testing: To determine whether or not the gateway would shut the solar down in a grid-outage scenario, where battery SOC was less than ~95%, and the solar production was higher than 5kw per powerwall (plus household consumption). We already know that the gateway shifts frequency to shut down the solar when there is a grid outage, and batteries are at 100%, but there was some question (at least to those of us who weighed in on this thread) as to whether the powerwalls might get shutdown too, since they can't handle a charge above 5kw per powerwall.
The test: Simulate a grid outage (by shutting off the main) during a time when solar production is higher than 5kw per powerwall, plus home consumption. To do this, I waited until shortly after noon today, and then shut off the main with battery power having been purposely depleted to about 61%. I also made sure that consumption was relatively low by turning off the heat pump, and making sure there weren't any other high draws. It has been a partly cloudy day today, with sun peaking out occasionally. Shortly before I did the test, the sun peaked out for a minute, and our solar was producing a little above 27kw as reported by the Tesla app. Clouds covered the sun again, but it looked promising that the sun would be back out soon, so I turned the main power off. Solar shut down for about 5 minutes when switching over to batter power, then came back on. So with the grid power off, battery power on, home consumption at about 1.5kw, maybe less, and the PV system producing, the full sun emerged again for about 20 seconds. I didn't get to see exactly how high the production reached, because the solar was shut down, but when it was sunny two days ago at the same time, it was producing over 28kw. So with 5 powerwalls, plus ~1.5kw consumption, there was a threshold of about 26.5kw. I'm guessing that that one burst of sunshine out the production over the threshold by 1 to 3 kw.
Results: The gateway adjusted the frequency in order to shut down the solar, but the powerwalls continued to supply power to the household (and outbuildings). I know that the frequency was adjusted because the error code on my SolarEdge inverters is code 64 "AC Frequency Too High (Max 1)".
So there you have it, folks, the powerwalls worked great, and I don't have to worry about high solar production frying my shiny new powerwalls during a grid outage.