I have managed the following:
From VC6, I have used Ole's DoDragDrop() method and the system global
memory to create a unique ID and drag it to Excel, which then triggers
a selectionchange event. In that event I call an add-in macro that
does a lot of things, including deleteing the unique id from the cell.
For the user, there is sometimes a flash of the unique id before I
delete it even though I delete it first thing.

Questions:
1 - Is there a better (no flash, more reliable) way to trigger an
event in Excel with the drag drop.
2 - Is there a way to trigger a permanent macro (in an add-in) in any
instance of excel from outside of excel, using COM or DDE or some
other communication method?
3 - Regarding the drag/drop, is there any way during the drag over
excel to change work sheets or scroll up/down/left/right
programmatically - e.g. like a mouse move event or a isdraggingoveryou
event in excel?

Thanks in advance.