[Fingers]

The pre-encounter condition is used in mantle_gar and mantle_url to set that race to be your enemy if you arrive without the mantle and have made friends with them earlier.

The preenc is triggered just before the radar screen would pop up. The radar would not pop up if they were friendly, that's why this feature was added. It may not be possible to use preenc to pop up a quest window as it's only intended to make (invisible) preparations before the actual encounter.

Strangely, there's no quest command to set a race to neutral. But if this is something you want to do on your first encounter with that race, why not just make them neutral in the race's ini file? (ie. not mark them hostile)