Originally Posted by Hyoss
Hyoss is correct that we can't override the popup menus. Unfortunately, PerformItemDrop will not work for this either. All PerformItemDrop does is allows us to move an entire stack within the backpack OR split a stack by popping up the built-in dialog that would have been popped up by simply right dragging the stack - it still requires user interaction to actually split. It does not allow us to programmatically set a quantity to split without user input so it does not work for the OPs problem of creating multiple splits automatically.
PerformShortcutDrop doesn't work either because the only way to drop a different quantity would be to create a new shortcut with the same generic itemID which would create a shortcut with a single instance of the item - the problem is, if you try to drop that shortcut you get an error because the actual item does not exist (it's not seen as one of the items from the stack).
So, there is no way to perform this with the current API (and that isn't likely to change at this point).