Quickly create/edit keyboard shortcuts for menu items
Here's the AppleScript for it:

on run

tell application "System Events"
    set theActiveApp to name of 1st process whose frontmost is true
end tell

set theApplicationID to id of application theActiveApp
set theMenuItem to text returned of (display dialog "Enter menu item name" default answer "Example: New Window")
set theShortcut to text returned of (display dialog "Enter shortcut (use these: ⌘=@,  ⌥=~,  ⇧=$,  ^=^)" default answer "Example: @$1")

do shell script "defaults write " &amp; quoted form of theApplicationID &amp; " NSUserKeyEquivalents -dict-add " &amp; quoted form of (quote &amp; theMenuItem &amp; quote) &amp; " " &amp; quoted form of (quote &amp; theShortcut &amp; quote)

end run


So, it gets the current application's identifier and then adds your menu item name and shortcut as required. You can even click the application's menu bar without the popup going away!

However, note that this is merely a rudimentary version of what you wanted; it only adds entries. If you make a mistake, that's yours to manually fix (and it might not appear in System Preferences). Also, lastly, it did not seem to work in Automator or Script Editor, but worked fine for the Finder. As for the former two, it adds the entries but for some reason they don't show up in System Preferences and can't be used, even after restarting the app. I don't know why but hey, at least it's something ;)