Since the old Halloween plugin didn't work how I wanted, I wrote a brand new one that has a list of maps to activate Halloween mode on and I installed it on the server.
Sadly, I installed it a little too late for it to work on BLU, as people apparently switched to cp_manor_event as I was writing it.
Edit: I can't say for sure that it enables everything. We just tested it on koth_harvest_event, and the crit candy was noticably missing. Likely, the Horsemann won't be there either. :/ However, the Thriller taunt and the halloween hats were useable.