Is it possible to set the page title and meta description through an app using the frontend app proxy?
This is for SEO purposes so I can't just do it with JS after the page has loaded.
I found some threads saying it wasn't possible but they were 3+ years old. I'm just wondering if anything has changed?
A framed page cannot affect the parent page, so this isn't possible still. You would need to use the page API (product API if it's products, collection API if it's a collection page, etc.) to change the SEO details.
I just realized how incorrect my response was. Whoops! Of course the APIs I mentioned won't be helpful for you to set the SEO details, as it's an app proxy page (/a/something, for example) rather than a Shopify object page. I believe you could do this through editing the theme using liquid. Let me look into it a bit and get back to you.