Figured out how to restart said Mongrel processes, but that didn't fix the issue. Turned out there was a database/MySQL error that was getting thrown back at the Ruby application due to the earlier lack of space. So it turns out, I had to restart MySQL first, then restart the Mongrel processes.
And I figured it all out on my own~ :: silly-proud:: =^_^=v
...and the log has hit 100 MB again, within 40 minutes of me getting it back up. Damn. I'm going to have to do something about this.