Merge mining leaves a signature in the coinbase transaction. A PPS pool which declares no blocks might be able to get away with it but no proportional pool could.
Yes but if a pool advertises it is merge mining 3 coins, but is actually mining 4 then it will hard for people to notice there's a 4th one there since there is a signature in the coinbase transaction for the others. They'd have to try and match up that signature with all known merged coins. A pool could even merge mine on a primary chain it doesn't advertise. Say, merge mine ixcoin, namecoin and i0coin with an unknown primary chain (a new one, devcoin, or some other). I don't know how users could track down what primary chain the pool is using.
Like I said if the pool is PPS then it doesn't need to advertise blocks so that is always an option. Then again in a PPS pool you are simply being paid a contracted rate per share. Who cares what the pool does w/ the hashing power. Technically someone could open a PPS merged mining pool which gives you an option of payout currencies. Totally hypothetical numbers:
1 BTC per 1,028,000 shares
1 NMC per 90,826 shares
1 IOcoin per 4,284 shares
or hybrid where pool pays out all 3 maybe even letting the miner set the % (i.e. 50% BTC, 25% NMC, 25% IOcoin).
In that case there is no deception. The pool is simply paying you a set rate per share. You can accept the offer or decline it.
The only "cheating" would be a pool which claims to be proportional and keeping part of the reward (hidding merged chains) to itself but that is very easily detected. Any proportional based pool is going to need to provide records of the blocks it mines and catching the pool is trivially easy. Remember the pool sends the block header to each miner on each getwork. If the getworks don't match the blocks reported by the pool well it isn't going to be long before pool's hashing power goes to zero (regardless of the reason). If a proportional pool refuses to provide records of blocks found it isn't going to survive either.
