[Paul M]

FYI, the 'show' check can use multiple variables, so if you want a link to only appear if $show['cond1'] and $show['cond2'] are set, you add them both, separated by a dot

<show>cond1.cond2</show>

Custom $show variables can be created pretty much in any hook, as the navigation is almost the last thing built, but two common ones added for the purpose are load_show_variables & load_forum_show_variables.