I've thought about PoW on transactions many times, but usually I end up thinking a 0.01 transaction fee is essentially similar and better. 0.01 is basically a proof of work, but not wasted. But if the problem is validating loads of transactions, then PoW could be checked faster.
A more general umbrella partial solution would be to implement the idea where an unlikely dropoff in blocks received is detected. Then an attacker would still need a substantial portion of the network's power to benefit from a DoS attack.
There, I said it.
Any demonstration tests at this point would only show what we already know, and divert dev time from strengthening the system to operational fire fighting.