Yes-and-no. Most pools already mine only against their own address - that is to say, the coinbase transaction only has
their address and nothing else. They then pay out the people using that pool from that address in secondary transactions. So yes, for them, it would be easy.
On the other hand, if they did that, they can kiss their pool (with all the nice and stable fees, etc.) goodbye. Nobody would want to mine there anymore. Their name would be forever tainted (which doesn't mean much in Bitcoin, but alright). So if a pool were to do something like that, doing it for 1-in-4 blocks wouldn't fly. They could do it only on outstanding balances, once; they'd better make sure it's a good one if they want to take the bitcoin and run
There are other pool types where the coinbase transaction is set up to pay out to the users directly, though - there's no cheating with that, as the pool operator setting it to pay out only to themselves would be immediately obvious (presuming the software is set up to detect it). P2Pool is a good example of this.