Hey B. R.,
Thanks for getting back to me. I am pretty sure that I was not able to make my point very clear.
The main point is that a client accessing www.example.com/items/1 is simply delivered a HTML file bootstrapping an AngularJS app. The JS client will then make a separate request to api.example.com/items/1 to fetch the data. At this point, if the API returns a 404, there is no way to send a 404 for the initial request, as the response was already sent. So my idea was to make a "precheck" request via nginx to check the item's existence prior to delivering the HTML file containing the JS client. Another way would be to have the server-side application deliver the client bootstrapping file, but that's not the way the architecture is currently set up.
Don't know if that makes sense. Looking at the documentation links you provided, I do not really know what you are suggesting as a solution.
Maybe these SO questions are able to explain it better?!
http://stackoverflow.com/questions/35989950/how-to-notify-googlebot-about-404-pages-in-angular-spa
http://stackoverflow.com/questions/37334220/how-do-i-return-a-http-404-status-code-from-a-spa
http://stackoverflow.com/questions/14779190/in-a-single-page-app-what-is-the-right-way-to-deal-with-wrong-urls-404-errors
Thanks again for your input.