I'm playing on IOS, started a couple days ago, and have no indication if she'll ever show. From what I've gathered she's just supposed to show up as a popup, but what are the actual conditions? Certain vault size? Was it a timed update so she won't show up anymore? Does she appear in lunchboxes?

Official info on this area is severely lacking...

Errr, that special freebie Piper deal has come and gone.

Now appears in lunchboxes. You may try your luck.

The chance of getting a legendary dweller in my vaults is about 1.5 %, and there are 22 legendary dwellers. I do not know the maths, but it is very hard to get it.

She is the best dwellers for mid game, because the high 'E' make it faster for training and get into action.

She turned up in one of my lunch boxes recently (droid)