The warpwallet challenge 2 here  you can find a public challenge aimed to brute force a brain wallet private key, derived from 8 char long [a-zA-Z0-9] passphrase with a salt of "a@b.c". Is it possible to adapt an hashing hardware to solve this? (Such as GPU)
Yes its doable, but you'd have to write your own custom implementation for scrypt and pbkdf2 hashing as warp wallet uses these 2 algorithms. If you can find a brute force tool for scrypt and pbkdf2 separately, you'd have to combine them somehow. Regardless, you'd have to code this up custom. That 20 BTC reward is worth 80k usd today.