Even if power is free (or "free") there is still overhead cost. So charging a % of mining revenue when mining revenue could be 0 or close to it sounds like a disaster. Another disaster would be if the fee is set to e.g. 50% of mining revenue and there is another crypto boom like last December. Sane customers would pull out possibly leaving the OP in the red with the warehouse lease etc.

Not to mention that it's unenforceable short of making everyone mine to OP's pools/proxies.