Is it not more likely that a big enough mining farm mines to a lot of different reward address, making it appear as many small solo miners?
What would be the incentive?
Keep in mind that a mining farm is also a solo miner but with a higher hashrate. Also, it is not just the address that indicates this is a solo miner it is the fact that the miner is not sharing the work with other miners and the work they perform is unique (on a block that is different from every one else).