SystemMenu includes source code for a system extension which shows how to install a global menu on the right side of the menu bar for System 8/9. It also shows how to use an icon instead of text for the menu title.

WARNING - Apple has never officially released information on how to do this, however code similar to this has been used in several shipping products. Even so, this source code is provided "as is" and you use it at your (and ultimately your customers') own risk.

Download the source code.