As I am working with collaboration diagrams on a regular basis, I’d like to see an implementation of the “message” symbol (the small letter attached to message flows) sooner or later. In my personal practice it is actually the “default” way to label message flows in case I need a label for them: I never label the message flow, but use a message symbol and label the business payload/content/meaning of the message.
I suggest to add (white “request” and grey “response”) messages to a context menu of message flows and let the symbol by default appear in the middle of the zone with which the message flow crosses the distance between the two connected participants. It would be necessary to label such messages of course, and it would be necessary to be able to move the position of the message across the flow and change the position of the label.