MacOS – Mavericks: How to ADD Input Source via plists/defaults

I'm trying to automate my OS X setup but I'm stuck trying to programmatically add a new Input Source or keyboard layout.

I usually use two layouts: regular US and US International, so what I'm trying to do is just add the US International layout.

Here's what I noticed so far. When I manually add the layout, I see this entry being added to my AppleEnabledInputSources key on the com.apple.HIToolbox plist:

--- a/def0.txt
+++ b/def1.txt
@@ -17,6 +17,11 @@
                 {
             "Bundle ID" = "com.apple.PressAndHold";
             InputSourceKind = "Non Keyboard Input Method";
+        },
+                {
+            InputSourceKind = "Keyboard Layout";
+            "KeyboardLayout ID" = 15000;
+            "KeyboardLayout Name" = "USInternational-PC";
         }
     );
     AppleInputSourceHistory =     (

I got this output by doing defaults read com.apple.HIToolbox. I can confirm the same addition by directly opening ~/Library/Preferences/com.apple.HIToolbox.plist.

So I decided that probably all I had to do was just add the same entry and it would all work. So I reverted the manual addition (and confirmed the entry from the plist was gone) and now tried:

$ defaults write com.apple.HIToolbox AppleEnabledInputSources -array-add '{InputSourceKind="Keyboard Layout"; "KeyboardLayout Name"="USInternational-PC"; "KeyboardLayout ID"=15000;}'

I confirm again via defaults read diffs that it adds the same entry to the plist, but it doesn't work!

I tried killing cfprefsd and SystemUIServer and even rebooting but none of these seem to work.

Does anyone have a clue about what might be happening here?