I'm sure that this can't scale at least against Google. They can fingerprint their own files. Might be interesting to check with MS or IBM services as a backup.

It's a CTF, so a scalable product was not the goal here.

Yes, I get it, but i need solutions and I'm considering in that regard.

I'm pretty sure it would scale for Google in terms of net profits ;)

Not if this decreases their captcha reputation.

Well, true, but Google's product managers don't appear to care at all about second-order effects