Using a PoW is something that I've thought about as an optional alternative to the captcha, though there's no need for it to actually do any altcoin mining.

Captcha-solving services charge about $0.003 per reCaptcha solve currently, so the computation cost to an attacker (not an average user) would have to be comparable to that. Certainly this makes SHA-x PoW impractical, as it would be far cheaper for an attacker using GPUs/ASICs and special code vs an ordinary user solving it via JavaScript on a CPU. Maybe cuckoo would work.

Another thing I've thought about is selling transferable blinded bearer certificates which could each be burned to solve 1 captcha.

But both of those would require significant development. If someone codes up the necessary libraries and end-user utilities, I would be very keen to use it on the forum, but I'm not going to create them (at least not anytime soon).

More realistically, I might sell one-time-use captcha-bypass codes at $3 per 1000 or something like that. This wouldn't be as flexible as blinded bearer certificates, but it might be sufficient.

Someone could right now write a userscript which integrates the forum captcha with a captcha-solving site. Some of them allow you to purchase captcha solves, solve captchas in exchange for credits (eg. solve on your computer and then use the credits on a mobile device), or transfer credits between accounts (eg. buy credits from other users). This is still not ideal from a privacy standpoint, of course.