After much head-scratching I finally figured out how to get a custom icon for the Main Menu on the panel.

For the benefit of anyone else who wants a custom icon:

yum install gconf-editor

Run gconf-editor and go to apps - panel - objects

The main menu is one of the "object_0", "object_1" items. Just go through them until you find the one that is the Main Menu.

Put a checkmark in the "Use custom icon" box.

Right-click on the "custom icon" entry (a few lines up from "use custom icon"), select "edit key" and enter the path to the icon you want to use.

That's all there is to it.