My guess is that the public library interface of GCC doesn't support it this way. This back-end uses the confusingly-named libgccjit (not as JIT), which gives

My guess is that the public library interface of GCC doesn't support it this way. This back-end uses the confusingly-named libgccjit (not as JIT), which gives access only to a subset of GCC's functionality. If something isn't already exposed, it might take a while to get patches to GCC and libgccjit accepted and merged.