This is exactly the same what I'm going to do with my own Office-style menus (search the forums for the code if interested). Remember though, that with toolbar/rebar-type menus you have to implement keyboard navigation of menus, which is a bit problematic. Here are the keys that should be implemented:
F10 -- for activating the menu.
Left and right arrows -- for moving between main item and menu sub-items.
Up and down arrows -- for moving between menu sub-items, down arrow also for activating a particular main item in the menu.
Alt-key -- for activating underscores.
Enter -- for selecting the current menu item, also expands sub-items in the menu.
Esc -- for cancelling keyboard navigation.
Implementing keyboard navigation is absolutely essential, because not everyone uses a mouse. And some people find it easier to navigate by keyboard even if they have a mouse.
Here is an article (MSDN) that explains keyboard navigation in toolbar/rebar -menus. Hope it helps.