> Fork Anubis. Slightly modify the hash function it computes. Deploy. Do not try to make your fork widely adopted. Do not even publish it.

A more robust solution would be to keep a few patches handy with different versions of the algorithm, and rotate which one is in use. This would keep the crawlers on their toes if they wise up to the changed algorithm. One could even imagine automatically rotating witch algorithm to use on a weekly basis.

It's not really defense by obscurity (the JavaScript is public), more like defense by... being different?

It doesn't matter what the hash is if it is inherently cheaper for a bot farm to compute the hashes than it is for a human to do it on their device. The human pays a greater cost in annoyance, wasted time, battery, and that means the PoW has failed its function. The bot farm owner does not care.