Why not just set a global text field as part of the startup script?
A global calculation is stored and will not re-evaluate without some trigger.
Why not simply type the user name symbol || on the layout?
Indeed. Comment has the correct answer. If this is only being used for display purposes then using the user name symbol is the proper way
However, most people do not really use the user name as much as the account name in which case one would set the global.