Use of the apparent difficult has been proposed many times before-- it and other schemes that result in a unique value for each block which is calcuable by the block finder immediately leads to a withholding attack:  If you know your block is unusually good, such that it is likely to win even if announced late, you are better off not announcing it.  The withholding profits are proporitional to your share of hash power (as thats how often your advantage gets you a second block) so it's also a centralization pressure.

Bitcoin's criteria has always been that each node adopt the first block it sees which meets the target, which creates an incentive to announce (one which breaks down if your share of the hashpower is over a large threshold, but if any party controls that much hashpower the system has other worse problems).