cstrombe Yep! That sort of routing is a fundamental feature of our system 🙂
Here's the basic idea from the Opalstack documentation for sites:
On the Opalstack hosting platform, websites are built by assigning domains and subdomains to sets of applications and URL paths.
This approach enables you to build any type of site from a simple static home page to a complex web application. For example a single site could have all of the following:
https://domain.com/ served by a static application
https://domain.com/blog/ served by a WordPress application.
https://domain.com/api/ served by a WSGI server running a Flask application
When you're adding a site or editing a site and click the "Add Application to Site" button, you'll see this:

You then select the app from the list on the left, then specify the URL in the text field on the right. You can add an unlimited number of apps (within your storage and RAM limits of course) to a single site in this manner.
The caveat is, as you noticed, all of the apps on a single site must be owned by the same shell user. Think of shell users as a "project folder" for the entire site.