the zeroth transaction of a block is the 50 BTC subsidy paid to an address you control. in the hash work the pool is handing out, that address is controlled by the pool operator, not you.
and you can't just substitute in your address, as that would result in a completely different hash, which is unlikely to meet the target requirement.
Yeah, I figured that out in my car in a "Duh!" moment. You know the transactions, since they give you the data, but you don't know the private key for the first transaction. Even if you do report it outside their system, they get the 50btc.
Oh well. Back to the drawing board. Lots of resilience to technical cheats, not so much social ones.