I suppose that, at first glance, you can't reference actions mentioned in service menus because they depend on a particular file, or set of files, being selected. Still, I'd have thought that it would be possible to select your files then use the menubar to trigger your action, although it doesn't look like that is possible with Konqueror in KDE 3.1.
I would expect some way of specifying your new menubar entries as a separate .rc file which can be referenced in the desktop file for your service menu. The trouble is that it would seem to only become "active" when the context menu is opened.
A plugin might be the only way you can achieve this. It would be interesting to discover otherwise.