so if you got a signed reply from 40-50% of the hashpower -
But how do you know something represents 40-50% of the hashpower? AFAIK, the only way of knowing is solving blocks.
For example, a miner could send all generated coins to the same address and sign the tx with that address. Likely a more elegant way exists. If you mean you don't know if they'll have X hashing power in the future, yeah, of course.