Inject hreflang header code into specific blog POSTS and portfolio pages?

I'm having trouble finding the answer to this question. I'm building a second location for a client website, and I need to insert a snippet of header code (hreflang tags) on a per-page, per-post basis, both in individual blog posts and individual portfolio pages. Of course there is nowhere to add injection code to these in the page settings panels.

Can I just drop a code block onto each post or page, and then label the hreflang code in some way that signals its placement in the header code?

Thanks for any help you can offer!

–– Cooper 

Have you tried edit each post/page > Add code via Code Block yet?