Thanks Tuan. Sqspthemes provided me with a script that work's for the entire summary block:
window.Squarespace.onInitialize(Y, function(){
openBundleLinksInLightbox('.summary-title[href]:not(.lightbox-link),.summary-title-link[href]:not(.lightbox-link),.summary-read-more-link[href]:not(.lightbox-link),.summary-thumbnail-container[href]:not(.lightbox-link)');
I'm using Lazy Summaries with this plugin though, and annoyingly the lightbox only seems to work for the first 30 posts in my summary and not those loaded by the Lazy Summaries plugin. Is there an obvious way I could get this script to read all the posts in my summary and include those that Lazy Summaries have included?