After using the out of the box AEM journal/blog component to create several posts on a page, when I click on a post to view the entry_topic page of the post, is there any functionality available to be able to modify the entry_topic.hbs file to include next/previous links to link to the next or previous blog posts?
I don't think, you will be able to maintain the info about prev or next post, once you have rendered entry_topic. Looks unlikely to achieve this unless you write a really inefficient code for server side extension to include prev and next journal's link in your entry_topic object and then extend hbs to include them in your UI. You can take a look at code at  to figure out, how to begin. Inefficient because, you will have to update the prev and next properties in every ugc in case of deletion of entry_topics.