To be honest, right click disabling can be bypassed so easily that it's hardly worth doing. Instead I'd encourage you to add a watermark to the images which makes it much more time consuming to abuse your images.
But if you do want to add a right click to hundreds of html files, you are going to have to make a copy of LR's built-in HTML gallery which you seem to have found.
Paste this default_html.lrwebengine in your application support folder (which has the presets and templates) in a folder called Web Galleries, which you may have to create. Rename the default_html, and then open the file galleryInfo.lrweb in a text editor. Change the title and id lines to something different from the built-in HTML gallery. For example:
title = LOC "$$$/AgWPG/Templates/HTML/Title=Lightroom HTML Gallery JB modified" ,
id = "com.adobe.wpg.templates.beardsworthMod",
You can then add your code to the header.html file.
Restart LR and you will see your modified version of the default HTML gallery.
John