An update to the saga - my 2000 EVC is at the mechanic, who just did a smoke test where they pressurize the evap system and gas tank with smoke. They have found a small leak which appears to be coming from on top of the gas tank, but were unable to determine the precise source of the leak.
The next plan is to drop the tank and do more testing.
This finding seems consistent with the symptoms: if there is some sort of small crack in one of the plastic or rubber parts, it could leak only when the pressure difference is extreme (e.g. heat + altitude, possibly made worse if the N-80 or evap canisters are clogged, or the gasoline has a high RVP).
I'll report back when I hear more.
