🚀 go-pugleaf

    I would like to use the "back end" of GNU CC to take some of the code
    generation load: I want to convert an existing Napier88 compiler to
    produce RTL, the internal register transfer language, and use the GCC
    back end to convert this to assembly language.

This is not the right way to do it.

Storing internal forms of a program in a file is a bad idea.  This
guarantees slow compilation.  Especially if the internal form is RTL.

GCC was designed to use RTL internally only.  Correct RTL for a given
program is very dependent on the particular target machine.  And the
RTL does not contain all the information about the program.

GCC was designed to support multiple languages.  It has an interface
specifically designed to go between the language-specific front end
and the rest of the compiler.  See tree.h for a description of it.