So... if a miner had 51% of the network, couldn't a "bad" mining pool simply set the ID on half of their miners to be "no ID" - so then it would look like they only controlled 25% of the network... when in fact, they would control over 50%.

Which would be effectively the same as the current situation where GHash publicly controls 40% of hashrate - and I've read claims (not saying true or false, just that some people claim) they also control another 10-15% of "unknown" miners.

Sorry if I've misunderstood.