I see what you mean now. Sending hashes for the target block that are a low difficulty. Well that could work, but I see 2 issues immediately. The first is that these hashes may or may not be produced in a regular manner, and there is not even a guarantee a client would produce one. The calculated hash rate based on this metric would have to be taken from a large sample size. The second issue is that each miner would now need to check the calculated hash against the target after every hash. This is another operation the client must perform and it will slow down generation.
I bet you'd get a good approximation of hash rate if clients submitted their best (highest difficulty) hash every N minutes. Over a period of a few hours the average of all of those best hashes should be proportional to the client's hash rate (unless a client were somehow repeatedly very lucky or unlucky, but that would be extremely unlikely).