Something came up today during a game. Does anything prevent me from using Har Ganeth to target units in discard piles as long as they less than 2 hitpoints? I was using this to get back shades and walking martyrs at certain points.

Couldn't find anything to back it up, but I feel like Units in the discard pile have no HP stat, thus wouldn't be eligible for targetting by Har Ganeth. Stand Your Ground's wording allows it to target a D Unit that entered the discard pile this turn.

Well, units in the discard pile still have resource cost, why not hit points or power icons? Maybe this should be answered by James.

No card can target an out of play area unless it specifically says so in it's card text, so no.