Broadcast WM_USER message

I would like to do something similar to BroadcastSystemMessage, but broadcast a message of type WM_USER+whatever.

BroadcastSystemMessage won't support message types above WM_USER. It says the user must do "custom marshalling". Does anybody know how to do this?

The only reason I can think of that it can't be done would be is another application responds to the same WM_USER message in a different way. I still would like to take my chances on this one.

MSDN reference: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/windowing/messagesandmessagequeues/messagesandmessagequeuesreference/messagesandmessagequeuesfunctions/postmessage.asp