If you want to assign a shortcut to the submenu you can just use the name of the submenu as it appears. So if I wanted to assign a shortcut to the default encoding in Safari, using "Default" is enough.
If however you want to assign a shortcut to a submenu that occurs multiple times, such as "My Folder" you can use: >Move To>Notes, in Lion at least. This no longer works in Lion.
You could also try to assign a shortcut using apple script like this:
tell app "System Events" to tell process "Terminal"
click menu bar 1's menu "Shell"'s menu item 1's menu "New Window"'s menu item "Pro" end tell
and then use these steps to generate an automator action:
- Open Automator and choose the Service template
- Add a Run Shell Script or Run AppleScript action
- Check that the values for accepted data types and application scope the are correct
- Paste the script and save
- Assign a shortcut in the Keyboard preference pane
If the shortcut for a service doesn't work at first, try running the service once by selecting it in the Services menu.
Credit for the automator answer goes to Lori,