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.
If miners (or at least large miners signed blocks w/ a private key then by looking at block signatures over the last say 7 days would give you a decent estimate of hashing power. Of course today no miner's do such a thing but if having X hashing power (that could be provable in realtime) was worth some economic value it would be relatively easy to implement.