When I play Mordekaiser, Dominating and discard Forbidden Technique to kill a unit, can the act of playing it immediately trigger the condition to use that Forbidden Technique from trash for 1e2p?

No — discarding Forbidden Technique as the cost of Mordekaiser's play effect doesn't work as a trigger, because Forbidden Technique isn't in your trash yet when the "played a card with Energy cost 7 or more" moment happens.

Walk the order: Mordekaiser gets finalized and enters the board (that's the play, and his printed 9 Energy is what the 7+ check would look at, Rule 420.4.b / 206), and only then does his "When you play me… you may discard 1 to kill a unit or gear" play effect go on the chain and get finalized — the discard is a cost within instructions, paid at that finalization (Rule 383.4.a.2, 383.3.b.1). By the time Forbidden Technique lands in the trash, the play event that would have triggered it has already passed, and Forbidden Technique's ability only exists while it's in the trash (Rule 385.2).

If Forbidden Technique is already in your trash when you play Mordekaiser, then yes — it triggers off his 9 Energy cost, you discard some other card to his effect, and you can play Forbidden Technique from trash for [1] plus 2 power (order the two triggers however you like, Rule 383.3.d).