I think there’s a way to override plugin templates with a theme component. I haven’t tested this, but try replacing the two script tags in the code that @awesomerobot shared with these:
<script type="text/x-handlebars" data-template-name="javascripts/topic-list-header.raw">
<script type="text/x-handlebars" data-template-name="javascripts/list/topic-list-item.raw">
Based off of this post: Who's Online Plugin (discourse-whos-online)