Overriding a theme function in a module

UPDATE: The comment by jtsnow resolves the general problem of overriding theme functions in a module using hook_theme_registry_alter(). Woohoo! But the specific problem described below to override the rendering of menu items remains.

I am trying to instruct the menu system to display each item's description underneath the item text, in a smaller font. I located the theme_menu_item() function in that currently performs this task.

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