How can I write to Notification Center from a launchd command?
You can send to Notification Centre using Applescript (osascript )in you shell script.

There is a very good thread here that explains the various ways of either running only Applescript as a shell script or being able to mix applescript in with bash, for example.

I created a shell script with:

#!/bin/bash
theDate=`date '+DATE: %m/%d/%y TIME:%H:%M:%S'`

osascript -e "display notification \"$theDate\" with title \"test 1\" subtitle \" sub 1\""


I then created a LaunchDaemon in the same way you have and loaded it without sudo.

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;Label&lt;/key&gt;
    &lt;string&gt;notifTest&lt;/string&gt;
    &lt;key&gt;ProgramArguments&lt;/key&gt;
    &lt;array&gt;
        &lt;string&gt;/Users/userName/Documents/notify.sh&lt;/string&gt;
    &lt;/array&gt;
    &lt;key&gt;RunAtLoad&lt;/key&gt;
    &lt;true/&gt;
    &lt;key&gt;StartInterval&lt;/key&gt;
    &lt;integer&gt;90&lt;/integer&gt;
&lt;/dict&gt;
&lt;/plist&gt;


Every 90 seconds I get a notification:

 

The notification icon you will notice is the Script Editors one. Which means the Notification is controlled by the Script Editors Notification settings in Notification Centre