ETA: owg, can you please tell how did you protect your forum? 
Very little actually, and it is probably just a coincidence, but here it is:
The login bot attacks started about a week or so ago. At the time, I had http:BL and Stop Forum Spam installed. About 4 days ago I installed CrawlProtect and Forum Firewall - I also had the list of the Tor IP addresses that someone posted in .htaccess. None of these measures halted the login bot. The pattern seemed to be more hits at night, and periodically during the day. Coincident with the attacks, I was being crawled by the GoogleBot in the address range 66.249.71.* in a way that I never have before. Typically Google visits my site during the day with only a single crawler IP, but now it was sometimes 20-25 simultaneous connections continuously during the day.
Because I thought it was possible that someone was spoofing Google, I went into Webmaster controls and reduced the number of times GoogleBots should visit, but there was no change in activity. Finally in desperation, I added that particular IP range (66.249.71.*) to .htaccess and the I watched as the failed login attempts dropped off one by one. This was yesterday morning, and not a peep since. I had even removed all of the Tor IP addresses that I had in .htaccess, which now contains only a single IP range: 66.249.71.*.
What is interesting is that Forum Firewall visitor logs reported a hack attempt by 66.249.85.3 within moments of my adding the 66.249.71.* range to .htaccess.
I don't know anything about security, and all of this is probably just a huge coincidence (the login bot probably just went away), but I'm just happy that my forum activity is back to normal.