Having a stable menu bar is a pretty strong convention on the Mac. If you didn't find any application that does what you want to do, you should seriously ask yourself if breaking the user's expectations about the platform is really a good idea.
If you look at native Mac OS applications, you will see that the contents of the menu bar are not changing but there is often one or two menus that are only active in some contexts. For example, in Mail, the “Format” menu is visible at all times but most of its items are disabled unless an editing window is activated. Conversely, most choices in the “View” menu are not available when writing an email. Same thing with “Select”, “Filter”, etc. in Adobe CS (Photoshope, Fireworks…) One major exception is Adobe Flash CS, which does have a number of menus hidden until you open a file.
Some menus might also simply provide functions operating on another level in the background. For example, in Mail you can synchronize your mailboxes at all time, even from an editing window. I don't really understand why an “Application” menu with options to open or create a new file shouldn't be available when other files are opened (incidentally, on the Mac, those two options should in fact be in the “File” menu, not the “Application” menu). Even if you can't have two files opened at the same time, having to close a window to be able to see the menu you're looking for can be annoying.
Going for a fixed menu therefore seems the best solution to match Mac OS users' expectations. If needed, there a couple of ways complex Mac applications reduce the number of menus: Move functions to toolbars, group them in sub-menus or secondary windows accessible from the menu.
Some applications do break this convention but those are often cross-platform applications and they do not really feel like real Mac applications. In the case of OpenOffice, the issue is that OO packs, at least conceptually, several applications into one. By contrast, Keynote or Excel look like separate applications, can be closed and opened independently, live in separate windows, with a specific icon in the dock, etc. If your application can be split up that way, that might also be an interesting approach.
You can even find some Mac applications with a menu directly in the window (often those are open-source projects or java applications), which might be a solution if you really don't care about the platform's convention and need to follow the MS Windows interface model closely. I suspect most Mac users would find them awkward and that could reflect negatively on the image they have of your company but such applications do exist (Matlab is an example of a large commercial application designed that way, presumably because the Mac OS platform is not its primary target).