Thanks for everybody's responses so far.
We're pretty confident that the crash caused the tire to unseat, and not the other way around.
We just wanted to see if others had similar experiences with (various) tubeless setups, or if this was a rather isolated incident cause by some extreme (but unlikely) lateral leverage at the tire/rim interface. Anyway, with only a 6" section coming unseated, this incident was nowhere close to rolling the tire off the rim. And it still took >40 sec for the tire to go fully flat.
Rightly or wrongly, I followed the advice of others on this forum (including one of the moderators) when selecting between the 'race' and TCS (tubeless) versions of this tire to pair with our Stan's Grail rims.
-"Stan’s specifically recommends NOT using WTB tubeless tires. I’ve been running the Nano 40c race version tubeless on Stan’s Grail rims for a year. It sets up perfectly and have never burped them. Running at 30 and 33 lbs for gravel and moderate single track. My weight 150 lbs. Have several friends with the same setup and equally good results."-"I’ve been running the standard (non TCS) Nanos tubeless on Stan’s Alpha 340 rims for several months now with complete reliability and very easy setup. The weight is much more competitive and I am very happy with the performance."
-"Mated to any of WTB’s suitable TCS rims, you have the first, and thus far only, “system” for tubeless tires specifically aimed at gravel road riding. What is more, these tires do work on HED Belgium+ rims and Ardennes+ wheels, and likely other tubeless rated rims for road and mountain use. Beware of mating these with Stan’s rims though, as this is not a good fit."
-"...WTB TCS tires do not fit well on Stan’s rims, as Stan’s rims are meant to fit NON TUBELESS TIRES and convert them to tubeless use. (This is why many have been confused with this review, thinking that the Nano 40 does fit Stan’s rims, which is true if you are using the NON TUBELESS Nano 40’s.)"
-"...Stan’s rims interface with the folding bead type Nano 40, (and most other folding bead tires that are NOT TUBELESS), as that is what Stan’s main purpose in design is for."
-"...From the Stan’s tire page, ” Not Recommend Tires. WTB TCS and UST 29 inch tires are too tight for Stan’s Notubes rims"
-"I have tried all three versions of the 700c Nano. If you have Stans rims, go Race or Comp."
-"I use the non-TCS version set up tubeless (TCS Nanos weren't out when I purchased mine). I keep pressures low, under 40psi. I have never had a burp, blowout or even a minor leak and I beat on these things."
Regarding field serviceability, I wasn't looking for a 'no-tire lever' solution with this setup, and I always carry tire levers and spare tubes with me (not CO2 though). While I can get these tires on/off without levers, it isn't easy to do so. I think this has more to do with the depth of the rim's 'well' than a loose bead. It also takes ~35 psi to get them to fully seat, and they snap loudly when they seat. I use a compressor to go up to about 30psi and finish seating the tires with a floor pump, so i can control the pressure.
Also, I have only used one 'non-field serviceable' rim/tire combo: 3.8 Surly Nates on LightBicycle carbon rims. This combo was so tight that others have considered cutting the rim to save the tire or vice versa, as once the tire is seated it is EXTREMELY hard to get off, because the rim's diameter significantly increases between the sidewall and the well. Anyway, the one time I got a flat on that setup meant a pretty long and unpleasant (hypothermia-inducing) walk through deep snow. Not particularly interested in a repeat of that experience.