The Haunted Scrap can be earned year-round on any server that has
tf_forced_holiday 2 (2 = Halloween) set. However, the haunted gifts only drop during the Halloween event. When the Halloween 2011 update later this week, gifts may only drop on servers registered with Valve. This is going from a long conversation thread on the hlds_linux mailing list:
We're going to be launching some new Halloween content in the near future and wanted to give you all a quick heads-up. There will be random gift drops, but this year we are requiring that participants register their servers. If you choose not to register, you can still run the Halloween maps, but no gifts will drop on your server.
The easiest (and expected) way to get the Haunted Metal Scrap is to melee the Horsemann while he's doing his death scream 2 seconds before he dies. I'm not sure if Meleeing him at any other time will award the achievement.
The problem is staying alive and being close enough to melee him. The easiest classes to do this with are Demoman (with one of the charging shields) and Scout.
His death scream is heard at 1:18 in this video: