Well while trying to find how to do this, I stumbled upon the MAX Remotemanagement system tray. I have enabled it on my test rig for...wait for it...testing.
Though I can't send a note to my customer, the reason I wanted to was for a pending reboot. I found that MAX already sends that information and you can't miss it. But I also found how to give the client a choice to abort the action and reboot later, which is what I wanted the note for.
I found this website which shows the cool stuff to do using GFI RemoteManagement's system tray.
To enable:
Logged into MAX RM portal
chose the W10PE PC and right clicked on it.
Clicked on System Tray and changed setting to On
Added Menu Option 1 - Type: Local File - Label: ABORT reboot - path: shutdown.exe - Arguments: /a
Click OK.
You WILL need to inform customer about the system tray and the ability to use the link to cancel a reboot. The system tray icon looks like a life preserver and if you click on it all of your menu options will be there such as website, support ticket link, how to stuff links etc.
Once the user logs back into their PC the system tray is automatically setup and is ready to go. If the user sees the message that the PC will be rebooted by your IT service provider, they can go to the system tray and click the "abort reboot" and go along their merry way.