You have to distinguish taking xanax orally/sublingually vs. swallowing it. Seems like your losing sight of the mechanics involved. I"m not sure whether when you refer to "plugging it" you're planning on crushing it into a fine powder first...but that would be the absolute only way you would experience any effect. Think about it...there's nothing in your ass that's going to break down a pill at any significant rate, to enable it to be absorbed (this might work better with MDMA, but these are pills that are loosely, unprofessionally pressed, not the rock hard, coated pills that over the counter xanax takes the form of).
So, what, you're going to powder your pill, and then....what? It's not exactly going to be easy as pie to get a finger of powdered xanax into your rectum without losing a lot of it, in the first place. SO it's either that--losing most of your powder in an awkward, messy process---or just lodging the pill up there, where you can be sure it's just going to sit for a prolonged period of time before even beginning to break down, because it's a professionally hard-pressed, coated pill.
I think this whole "plugging" thing is more a fad than anything else..."let's plug this, let's plug that"...it developed in the ecstasy scene, and people don't think twice about the difference between that and the mechanics of plugging a professionally pressed, coated pill, etc.
If you just have to plug SOMETHING, then dissolve the pill in water, fill a balloon with it, or put it in a syringe (remove the needle first, mind you), make sure you have an friendly procto-consultant to assist so that you don't chuck the whole thing in your awkwardness, and fire away, I guess. Keep me posted about the effect.
But I can guarantee you it's not going to come on any faster than just chewing it thoroughly, and I'll be damned if I understand the whole romance behind plugging for the sake of plugging. And with xanax, that's all you're going to be doing.