Spoiler
With piety bug I think he refers to piety overflow. In other words pray and piss off your deity until the piety counter warps around and you get maximum piety

Spoiler

every time you pray your piety goes down a bit, depending on what the god does for you, it also goes down if you don't get anything from the god.
And then if you pray a lot the god starts to hate you and will curse your stuff and destroy it etc. but when you pray enough you reach the minimum amount of piety as possible in other words the god couldn't hate you more.
But if you then pray some more, the piety counter warps around and you get maximum piety.
I son't know the minimum and maximum piety amounts in numbers, but let's say it's something like this:
Minimum piety = -1000000
Maximum piety = 1000000
If you pray when your piety is at -1000000 the counter will warp around and go to 1000000 piety.

Spoiler

You need:
-about 150 HP (enough to survive Bolts of True Matter)
-A source of freezing
-A source of fire, to melt the ice (Burning Hands doesn't work)
-Some useless items you don't mind having destroyed
-A good supply of food
-The Old Barbarian led to a safe place, or to already have gotten Courage.

Best way to do it is to be Neutral or Lawful (greater daemons have fire breath that can reach you), then to freeze your way into the Barbarian's Glade river, and then melt the ice around you with fire. Be at the edge of the level, so that one step will take you out of the level. You now have a safe little ice island.

Drop everything, equip a single worthless item, and start praying until the item is destroyed by your deity.

Pick up another item, equip it, and repeat until that one is destroyed.

Repeat that until you get hit with a bolt of true matter, which will take off about 100 HP. Heal up, get hit again; heal up; get hit again.

After two or three bolts the god will start summoning creatures; and that is when you hit the "_" button and hold it down. Be sure you have a lot of food, and that you eat it when you need to.

When your deity stops summoning creatures, you will know the piety counter has overflowed. You are now ready to be crowned as many times as your level allows.

Your god's summonings are also easy prey for your Bolt spells; though you will have to be careful of your alignment if you are lawful.

If you've frozen the right spot in the Barbarian's Glade, you should be right on the border of the level and be able to leave it without having to fight your way through the summoned creatures.

Spoiler
Unfortunately, "holding down" the _ key seems not to work here - since more than one message is generated, you have to press a sequence including the space key. This makes it a very slow effort indeed.

Did you see the number of interventions you asked for in the ending dialogue? I got up to 150 at one point, but lost patience (and had no time)...

Also, I did this with a gray elven wizard - just darkening a room was enough to avoid the critters.