Shopify App injecting Liquid code into theme files

I'm developing a new Shopify app and need some Ruby on Rails dev help.

When a Shopify Merchant installs my app, we ask the Merchant to manually add some Liquid code to their Theme files.

But my Shopify app reviewer recommended that the app automate any code injection. He says I should use ScriptTags and/or AppProxies to inject code into Liquid files. Specifically, we create new /snippets/ files and need to inject the {% include %} tag into the product-template.liquid and customer/account.liquid files.

Does anyone have experience doing this? If so, I'd love your help.