The Windows Registry has a value which sets the marquee text. LabView 6 and 7 have Windows Registry Access VIs which you can use to write a string. Use a function from the String >> String/Number Conversion palette to convert your numeric value to a string, then write it to the registry.
Registry key: HKEY_USERS.DEFAULT\Control Panel\Screen Saver.Marquee
Registry value name: Text
See the attached LabView 6.1 example.
LabView ships with example registry VIs: Help Find Examples >> Search, then enter Windows Registry in the box labeled Type a keyword to find.