Can a promoted Joust minion use its Eager talent on a minion that is on a non-path hex?

Yes. Joust minions can use their Eager Talent on minions even if they are on non-Path hexes. If they do, they would follow the rules for invalid hexes, moving back to valid Terrain at their earliest opportunity. Note that Eager only displaces with minions, not all Units, so Joust minions do not swap positions with heroes that they Target.