This is just psuedo code, you should find and test for the actual variables to use:
Code:
{if $cat.categoryid eq 12}
{*special behavior goes here*}
{if $logged ne ''}
{*Show category*}
{else}
{*Show message to log in with link*}
{/if}
{else}
{*Normal category behavior goes here*}
{/if}