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.
So just in case anyone finds this through searching.
So in rails I created a layout template with this content:
app_proxy.html.erb
{%- layout none -%}
<%= @theme.value.html_safe %>
The first line is important because that tells Shopify to render the page without any of the normal layout.
Then in controller I load the layout/theme.liquid from the theme through the API so I can change the content of it. I do a simple find and replace on the title tag and something similar (but unfortunately not very “nice” because of the way it is in the template) with the meta description. Then I put my content in with the doing a simple find an replace on the main content liquid tag. I then make sure to send this all back to Shopify with the application/liquid header so that Shopify then renders the full page as normal. Something like this: