I ended up just using jQuery for this. Put it in my master page in a script tag.
if ( document.location.href.indexOf('<string in URL - I used the name of the document library') > -1 ) {
$('UL#topnav A.SecondTab').css('background-image', 'url(highlightedtab.jpg)');
}
Used several if statements because each tab has a different class name, e.g. FirstTab, SecondTab etc.