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.

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

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