Unspendable =/= Invalid. Unspendable is just unspendable. It could be unspendable on purpose ("burning coins" or by accident (wallet software errors) but it is unspendable all the same.
{
"height" : 305997,
"bestblock" : "00000000000000005c56379fc24b9b75c5c44c7afdb75f0c5b0801c56ec4f6bf",
"transactions" : 3305419,
"txouts" : 11452422,
"bytes_serialized" : 396365647,
"hash_serialized" : "10c14635b7ca03524099eb1bef0018480afddb67986b569fc9aa58bf1d16bb73",
"total_amount" : 12899789.79002854
}
To date 135.20997146 BTC have been destroyed.
(305997-210000)*25 + 210000*50 = 12899925.00000000 BTC
12899925.00000000 - 12899789.79002854 = 135.20997146
I have no idea what you are talking about. An unspendable output is an unspendable output no matter who creates it. Nobody said anything about a pool creating an unspendable output.
I was referring to this post about the pool claiming less then the 25 block reward
They do this by creating a special transaction that has no inputs, and assigns outputs that do not exceed this sum to any addresses (or scripts) that they like.
Notice, that I said they assign outputs that do not exceed this sum.
In other words, the protocol is perfectly happy to let a miner assign LESS than the reward that they are due in this transaction.
This has happened in the past accidentally. It is not likely to happen intentionally, but that doesn't mean it can't.