The monster twist is a cool angle to try out. Some suggestions, take 'em or leave 'em:
I'm just speculating, but if you worked up a Hero Scribe map of a section of the town and some Monster Cards for the players, perhaps it would serve as a draw. Some of the PBP threads have gotten very creative in display information.
Also, I think it would be a good idea to repost here the necessary
Death Match rules that apply to
Monster Siege!, as it has been a while. Shorter posts=more readers, so I'd say edit down the repost as much as possible. You're looking for a pool of players, so you'll need to make some compromises. Most HQ players prefer the familiar, simple gameplay over a set of new houserules. Switching most of the Hero rules to yourself is a good idea, so I think you can save a lot of space here only posting what monster players need to know. A briefer summary of what to expect from Heroes should be sufficient to get play started.
From our experience of play in the
Melee Tournament, I'd be ready for players to team up whenever possible against the "good" common opposition. Hero Quest is normally a cooperative endeavor for the players, so they are rewarded by sharing BP resources through unspoken alliances.
My opinion is PBP can can drawn out over time with all the delays. For that reason, I'd say a single board with players starting in each quadrant would be preferable to four boards combined. It would be clearer to post, and it would also bring about a resolution while interest is still strong--this isn't a Twilight Imperium board.

If you have more you want to test out, you could continue the campaign on a new board with either a fresh
Monster Seige! or link it like
The Dark Company.
Knocking out Heroes with white shields should prove time consuming, as
Melee Tournament has shown. In contrast, monster players throwing black dice should be expected to burn through thier rosters more quickly. I think it should prove helpful to playtest some combats at home to get a sense of the pacing, if you haven't already.
