IDEA: reverse genocide, or otherwise manage to get a hold of a wraith. Let it touch me all the way back to lvl 1 or 2, use my gargantuan amount of money at a local temple to buy incredible amounts of AC.
Firstly, I think if you can afford to pull this off, you don't really need those protection anymore. They will just be icing on the cake.
But anyway, here's what I can think of about the "reverse protection run" strategy...
1. Monster level
You are the same dungeon level when you are very high level... that means, there will be tough monsters that you cannot handle as your level drops. I think you need to do this in a 5x5 boulder fort. You will need E-words on the outside to scare away giants but not the wraiths.
2. Damage
A wraith's touch attack does 1d6 damage, so you will need some methods to heal. Potion of full healing will restore half of the loss XP, so you can't use that.
3. Breaking out
You will need to chat with a priest while you're low level, with the wraiths clawing at you. The ideal solution seems to be including the priest in your big boulder fort (5x5 plus space for a priest), a method to burn Elbereth and a way to kill wraiths so you can move next to the priest. Or just have the priest next to you and settle for 7 wraiths.
4. Teleporting monsters
They might teleport in and do something horrible to you, as you cannot have E-word inside the boulder fort. To prevent this you will need to keep all the wraiths alive to fully occupy the boulder fort, and burn E-word when you are done with the protection and breaking out of it.
5. Restoration
You will also want to get your XP back once you've bought protection. Potions of full healing can be used to restore half of it, but your level will still be much lower. I'm not 100% sure about this, but I think I've read somewhere that when you reverse genocide wraiths, the level will become a grave level (TDTTOE), meaning the chance of getting a wraith corpse is extremely low. So you can't eat the wraiths to get some levels back, and you will need some potions of gain levels.
6. Last but not least: Does it work this way?
I'm not sure if the protection cost is based on your current level or your actual level...
But hey, it's a creative twist of a protection run... please keep us updated on how it goes.
Does it matter if it's a cross-aligned temple?
You can get protection from any priests, even from priests of Moloch... they all have their greedy eyes on your purse.