How can I add a content box to the order status page from my App (and not via Additional scripts)

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Hi, I want to make a content box that appears, or does not appear, depending on logic involving liquid variables. 

Can achieve this all from within my App ? (so that this functionality comes as part of the app, without the need for buyers of my app to manually fill in Additional Scripts in the admin settings)

Basically, I want to display the content box ONLY IF a product is a DOWNLOAD.

I know that Shopify's own download app Digital Downloads does this - it displays a content box only if relevant downloadable products have been sold. I know (from the permissions) that their app does use Scripttags, but I understand that script tags don't have access to liquid variables. How would they achieve this functionality?