sorry, i wasn't clear.  i meant that you'd need to change the gnat code generator
to generate a different representation, not that you'd need another
code generator working from the RTL.  as you say, gcc's RTL
is machine-specific.

by `if gnat is structured so as to allow another ... in principle',
i meant, it has some internal representation of the Ada program that
is at or near a useful level for code generation, before it produces RTL.
if it is producing RTL directly from a basic parse tree,
you'll have your work cut out.

i don't remember dewar's description of gnat's structure,
except that it converts its internal representation of Ada
to RTL at some point, so i don't know the answer
and i can't be much more specific without looking
at Gnat's code (which i've also seen but forgotten as regards this point).
it was a good five years ago and i was curious
rather than interested.