I was also able to load it under LaunchDaemons.
I'm not sure but I'm under the impression that LaunchAgents require user interaction.
In either case it loads and unloads without a problem.
I'm unable to catch a PID using both the activity monitor and the console.
I changed the StartCalenderInterval and watched as it passed the time however currently there are no updates pending.
In what little time I had been looking into it, before I saw you revision, I had come across the possibility that it might need some form of the following:
<key>com.apple.private.AuthorizationServices</key>
<array>
<string>system.download.apple-software</string>
<string>com.apple.softwareupdate.manage_daemon</string>
</array>
Regardless, thus far everything seems great. Might have to add the KeepAlive or wait until there's an update. (experimentation needed)
I can see in the logs where it loads and unloads but the fact that it's not showing a PID makes me think although it's loading, it's not running.