This doesn't seem to be general reference, because in the context I've seen it used tonight (the London riots), it certainly isn't referring to a coin, or 'a person who hails'.
I've heard it used several times now by mainstream media reporters; they saw someone with a "loud haler". Here's an example from Twitter:
5 live producer reports seeing rioter with loud haler - calling for friends, seems very organised
Does anyone have any idea what they mean here by 'haler'? Even Urban Dictionary doesn't seem to have a satisfactory definition. My best guess is it's some London street slang for 'megaphone', but can anyone confirm?