Tom "711" did some hacking and we figured out that the following
settings will result in the correct answer:

Tools - Options - General:
Distance calculation = "using FAI spheroid" (should already be
set)

Task options:
Type = AAT
Task distance calculation = use flight fixes
Task time = 00:00:00

Observation Zones (all Fixed value or Symmetrical direction with
Angle1 = 180):
Start: "Move origin", "Reduce leg distance" -- doesn't matter,
because of "use flight fixes"
Finish: "Move origin" ON, "Reduce leg distance" OFF (only this
combination works)

The surprise in this solution is the Finish OZ settings -- the "Reduce
leg distance" option has opposite the expected effect.

Another contradiction to expectations is the "Task time" -- I expect
that to act like a minimum task time, but SeeYou treats it as a
maximum task time, so you have to leave it at 00:00:00 or put in an
arbitrarily large value, otherwise SeeYou "photo finishes" you at that
point after the start.

Thanks Tom!

tuN7o