Combo of Jackson (mostly) and Hartigan when he's resting - neither is ideal, but we don't have much to go with.
IMO, if you're going to tag someone, the tagger needs to be:
a) Largely expendable in terms of your attacking game plan (rules out Tuck IMO).
b) Physically capable of matching their opponent for height, pace and endurance (rules out most of the other possibilities besides Jackson IMO).
Having said that, Jackson hasn't really shown any natural tagging tendencies (yet), but if he loses the battle, we've not lost a lot...while if he comes off even we've won a huge victory. IMO, half the battle with a tag or any defensive match-up is negating someone you rate as a threat using a player of lesser standing. If you use a player who has attacking value to your own plan (Coughlan or Tuck on Goodes for example), then in many cases, you've already thrown up the white flag in terms of kicking a winning score.
Hence the value of Kirk, he can negate and still be a valuable offensive weapon...he's like gold for a coach. Several of our younger blokes have the physical attributes to be similar players...Jackson is worth a try, Hartigan is already showing potential, but Jackson has a height and athleticism advantage. No harm in trying this move in a game we're likely to lose...nothing ventured, nothing gained...that's how you pull off surprising victories...experiment a lot and develop fringe players who can play a variety of roles, hopefully to your eventual advantage.