attr_accessible does not change any thing. Maybe a bug in user_stamp gem?
userstamp doesn't respect the namespace and the default stamper_class_name is User but alchemy_cms' user class is Alchemy::User. So the quickfix would be to change the calls to stampable to stampable(:stamper_class_name => 'Alchemy::User'). I tested it, it works and I could provide the patch. The other possibility would be to "fix" userstamp, although IMO it's not really a bug.
stampable(:stamper_class_name => 'Alchemy::User')
Set the stamper_class_name to Alchemy::User.
This fixes an issue where the creator_id was not set.