The SeaMonkey guys seems to be doing exactly that (backporting engine changes), albeit with Gecko instead of Goanna, and by freezing the version of Rust in order to allow for Windows 7 compatibility. They weren't able to remove all dependencies due to lack of manpower, but I suspect they may have done some work, knowing how at least some of their team tend to operate*.
I'd hop into their IRC Chat if I were you. I honestly wish they'd get more attention for what they're doing, especially around these parts - and particularly with how they were shafted by Mozilla themselves and effectively kicked off their infrastructure around the FF Quantum transition period. That shit was painful.
* One of them literally made this... thing, by the way. |