How to make truly single spaced bullet items in Keynote

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.

The substitutions for Pages.app are stored in: ~/Library/Preferences/com.apple.iWork.Pages.plist

Specifically they are in Root/kSFWPAutoSubstitutionArrayProperty. The format is a an array containing dictionaries with the keys replace, with and on.

The Global Substitutions are located at ~/Library/Preferences/.GlobalPreferences.plist. Specifically, they are in Root/NSUserReplacementItems. The format, conveniently, is identical.

It's probably possible just to open ~/Library/Preferences/.GlobalPreferences.plist as plain text and copy across what you want into ~/Library/Preferences/com.apple.iWork.Pages.plist.