My original thought was that the account creation page (or whatever needed to be gated) would do the mining and send the server a small amount of e.g. Monero. A

My original thought was that the account creation page (or whatever needed to be gated) would do the mining and send the server a small amount of e.g. Monero. And alternatively, if you didn't want your device to actually mine the Monero, perhaps because you don't allow scripts, you could instead have your browser send that small amount directly from your own wallet. :-P ...but people have browser extensions to sniff out miners now, so a more generic proof of work is probably safer.