I am trying to print a message on the desktop when the user has logged in without modifying the core, ie from a separate app, could it be done?
You can achieve that by creating a Note.
Tools > Note > New
If you want it to be seen by every user, you can make it public.
There is also option to Notify users with a popup when they log in.
No, that is, when you log in make a frappe.call and according to the request show a msgprint.
I am looking for the same functionality as well