Why not charge Tor and VPN users a small bitcoin fee to log in? Most of those users would probably rather pay a fee than use cloudflare. They already have to pay a fee to register.
A fee
to log in!? Are you serious?
N.b. that (a) the move behind Cloudflare at the end of November is absolutely irrelevant to
login issues, discussed separately since October; (b)
everybodys connections go through Cloudflare, for every connection to the site; and (c) Tor users (among others) are already charged a fee
to create an account.
Theymos said he's unenthusiastically using Cloudflare to protect against DDoS attacks. I assume some of those attacks come through Tor and VPN users. Those users couldn't DDoS if they had to pay a tiny fee to login, and further fees if they make excessive HTTP requests. They are already prepared to pay a registration fee for privacy, charging small log in fees isn't much different.
Furthermore, charging a fee for excessive HTTP requests could protect against botnet DDoS attacks from regular IP addresses. Normal users wouldn't even notice because they don't make huge numbers of HTTP requests.
I guess most of the accounts involved in DDoSing are newbies. During times of excessively heavy load on the forum newbie accounts could be asked to either pay a small log in fee, or return later when there's less users accessing the system.