Hmmm... if you need to go deeper (because 1/4 of all hashes have zero leading zeros and zero trailing zeros), you can generalize this by converting the hash into its run-length encoding to get a sequence of rank functions where finding values with the same rank for all rank functions is equivalent to finding hash collisions. Very nice.

Whoah, I totally hadn't taken the thought experiment this far. This is fantastic! I'd like to explore this further, interested in a quick research chat/sync some time? My email is linked from the paper.