Mining is typically something like: Hash(nonce + Hash(chain[including with new transactions])) < value. So in some sense you need the chain but you can pass H

Mining is typically something like: Hash(nonce + Hash(chain[including with new transactions])) < value. So in some sense you need the chain but you can pass Hash(chain) to a client and let that just increase nonce and hash it. Without synchronisation the found block might be for a stale chain but if you're using someone else's power you don't care, and the nature of distribution here means it's likely to be fresh anyway in the unlikely chance something is found.