Does Mask of Foresight work on an open showdown?

No, Mask of Foresight does not work on an open showdown.

According to the RiftJudge FAQ (ID 1368), Mask of Foresight only activates during combat, which occurs when units from two players are present at a battlefield. It does not trigger during an open showdown (when you move to a battlefield without any enemy units).

Furthermore, as noted in RiftJudge FAQ 4707, attacking and defending only occur when there is combat; moving a unit against an empty battlefield does not constitute "attacking" or "defending" in the rules, so the trigger condition for Mask of Foresight is never met.