I'm looking for this too, and the other poster's suggestion is not complete - but one thing you can do (it isn't the solution exactly, but it might be nice for you) is to set one of your 'hot corners' to show the desktop - I use the upper left. This is done through the Screen Savers system preference panel. Another solution: using something like Keyboard Maestro, you can assign a macro shortcut to the action Hide all Applications or, if your macro program doesn't have that, you could simulate it by having it click the desktop and then type the key Command- Option- H (which is a Mac OS standard shortcut for hide all apps but the foreground one). Something I'm planning on trying is, which claims to hide applications after a certain idle time. Not the whole solution, but perhaps, over time, will achieve the same result.

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I'll report back here if I find the EXACT use case solver.