If you send somebody out in the wasteland for long enough with good enough luck, is it possible to find power armor or any other legendary gear?
Yes it is possible, I have found Legendary outfits and weapons while exploring the wasteland. It is definitely easier to find gold gear in lunch boxes, but it is still possible to get really lucky while wandering.
To be specific, I found a "Heavy Combat Armor" (4S 3E) in one vault and an "Tuned Alien Blaster" (18-20 dmg) in a different vault.
I have t45d power armor, heavy mercy gear, heavy wasteland gear, smugglers end, accelerated railway rifle, and a rusty plasma rifle in my vault that has less than 20 dwellers. However I have been increasing my dweller count slower than normal so I can properly equip all my dwellers. All of these except one were achieved in the wasteland without reaching the NGD.