OnMyCommand allows commands to be added to the shortcut menu. Unfortunately, I can't add them to the services menu or add a keyboard shortcut for them. Is this possible?
Adding on the command to services menu or adding keyboard shortcuts
keyboardservicesshortcut-menusoftware
Related Solutions
You may want to try Services Manager.app:
Download the application and put it into your /Applications folder (or any other convenient place). After you started it you can configure each service to appear in the Services menu (by enabling it in the Active column), in the Context Menu (by enabling it in the Context Menu) or in both.
This doesn't solve the general case problem of how to assign a keyboard shortcut for any dynamic menu item, but it does solve the specific problem of having a keyboard shortcut to hide or display slides in Keynote.
We can work around the shortcut problem because Keynote is AppleScriptable.
Create a new service in Automator.
The service takes no input in Keynote.app.
The service has a single action: Do AppleScript. The script (in all its one-line beauty) is:
tell application "Keynote" to tell current slide of the front slideshow to set skipped to not skipped
Save the service as "ToggleSkipped".
Use the Keyboard Shortcuts tab within the Keyboard pane of System Preferences to bind the service to an unused keystroke.
Best Answer
Use Apple's automator and save the result as a service. If you select the right conext for the service it will appear in the right click menu. You can add shortcuts to services using System Preferences > Keyboard > Keyboard shortcuts. Click somewhere to the right of the service name (near the right edge of the white box). A field appears and you can type a shortcut. This is not very intuitive.