Maybe an GCC/EGCS, eglibc/glibc, or (as nfm points out) Yarv/MRI scenario could work. First prove your mettle (will take donkey's years), then upstream can jump to your stable code base in one glorious commit.
Hope you can mention in the readme that upstreaming is not a goal... It might be hard to get the wording right but it would have made my first impression quite a bit less skeptical. :)