Theming menu-link under some circumstances

Hi,

I'd like to theme menu-item-link, to display different in book module. (Let's say add thumbnail icon of subpage node into navigation).

But obviously, I don't want to change menus at whole page look, but only inside book.

I thought about something like that:

function phptemplate_menu_item_link($link) {
     if (preg_match("/^book-toc-",$link["menu_name"]) {
          //load node, display node image instead of text
     } else {
          return call_a_theme_function_which_would_be_called_if_havent_theme_it($link);
     }
}

Is something like that possible? Calling a "parent" theme function under some circumstances?