Dear Fabian,
Technically, we could pull this off quite easily. If we agree on an identifier and a structure of the menu, the first plug-in that would see the menu missing could create it (we would do it on the main thread, so no need to worry about race conditions).
But I am not a big fan of having stuff like version numbers in menus. What about adding just a single submenu: "Plug-ins" (or "Plug-in Preferences") and having a single item (that would open the correspodning preference pane) for each plug-in? Separate preference panes are already an established way to display Mail plug-in interface. This is also where I show my version number and the removal option.
For me the question is whether it is worth it to essentially reduplicate the interface offered by the Mail preferences window. (It would be great though, if someone could make a better toolbar for it that would show all items in multiple rows so that we would not have to explain to users that they have to use the disclosure arrow:).)
--
Adam Nohejl, Lokiware
http://lokiware.info
twitter: @lokiware