If you do call it a "first volume," make sure to put the cutoff right after Ch. 63 "TSPE Aftermaths" if possible. It'll make more sense that way because its the end of an arc.
I am in favor of the first volume being standalone, period. I'd rather chapter 64 be listed as chapter 1 of a second work.
This year I'm a supporting member of WorldCon for the first time, and I noticed that the Hugo's have a category for Best Fan Writer ("Any person whose writing has appeared in semiprozines or fanzines or in generally available electronic media during 2010."). I do believe Harry Potter and the Methods of Rationality counts, and I plan on nominating it. Even making it onto the ballot will probably expose it to a wider audience. Is anyone else here a WorldCon member and thinking of nominating MoR?