Syntax for Submitting Subrequests to FastCGI

Is there a syntax to submit subrequests to the FastCGI backend?

For example, if you want to use js_content to do some work before the content is retrieved, issue a subrequest to a FastCGI backend, and then do some work on the response (that can't be done by FastCGI and what it hands off to), then a subrequest to FastCGI would be needed to do that.

Right now, the documentation seems to imply that subrequests only work for proxy style locations. But is there a way to route the subrequest to a FastCGI backed?

I've tried using a subrequest to a named location, but that doesn't seem to work (it only works for internal redirection). Is there something that is supported like the below as a URI?

fastcgi://path/to/socket
fastcgi://ip:port

We use socket mode if that is relevant. But ultimately, I'm just looking for a way to issue a subrequest to a FastCGI backend.

Start by reviewing the njs entry points for js_content and subrequests, then compare how named locations and FastCGI-backed locations are handled. Determine whether a supported way to issue a subrequest to a FastCGI backend exists; done means the syntax or limitation is clearly documented.