If the intention is to make him 'untargetable', then why is Nunu missing his giant 'no-need-to-target' aoe spell?
I know for a fact if you time it right you can, with Brand as a great example, hit Maokai mid Twisted Advance and do damage to him with Pillar of Flame and/or Seer. In the same respect you can hit Fiora mid ultimate (name eludes me right now) and the few other champs I can't think of right this second as well.
If the intention is 'untargetable', then Maokai, so long as he is still in Nunu's Ultimate, should still take full appropriate damage. If the intention is 'un-damageable', then he should take no damage at all.
On a side-note, if he is 'un-damageable' in Twisted Advance, this would be the equivalent to a mini Kayle ultimate.
Also on a side-note, if you have Brands passive already applied to Maokai before the advance, he does his advance, you Seer him before he finishes traveling and lands, the stun application will still take effect but ONLY after he lands, and with the timer of what is left of the stun factored in after travel time. This also does not prevent the root, but does prevent any action done by Maokai for whatever duration is left of the stun. Trying to simply this here, Brand lands Pillar of flame, Maokai Advances, brand lands seer, thus stunning him, if it take 0.5 seconds before he finishes his Advance and roots his target, and the stun lasts for 0.5 seconds less then full intention even though it TECHNICALLY should last the full duration after the Advance is complete.
Just my two cents, hope it helps.