This is an idea we're kicking around for a MET implementation of Fur Gnarl. It strikes me as a) expensive and b) still totally worth it in a lot of circumstances, which means it's about right.
Step 1 - Make a hole: Spend physical traits equal to the levels of DR you're trying to remove. Then make a regular close-combat challenge versus your target. Succeed, and your packmates (and you) can move on to step 2. Fail, and you (or someone else) must re-attempt step 1. Either way, you've spent some traits.
Step 2 - ???: https://www.youtube.com/watch?v=tO5sxLapAtsStep 3 - Profit: Any packmate can make a physical challenge to attack the weak spot. Bid extra traits equal to the amount of removed armor as you attempt to hit the weak spot. Succeed, and you deal DR-ignoring damage. Fail, and you've lost an unusually high number of traits.
Caveats: Ranged weapons can't be used in step 1. You are your own packmate, and may use this solo even if you have no pack totem. Like all pack tactics, you must have the tactic on your sheet in order to use it. Crafty opponents may take actions to hide their weak spot from you by putting it up against a wall or something. Anything that causes your target to heal significant damage (Mother's Touch, multiple rounds of healing without weak-spot attacks) is likely to close the hole on targets that have inherent armor, like scales.
Once we finalize our house rule, anyone wishing they hadn't bought it will be allowed to un-spend their XP on it.