If it keeps working in FF 57 and won't break after every minor update, I might upgrade. I was actually thinking that providing some kind of officially unsupported backdoor for determined users like us might be a way out of this situation.
The Mozilla-supplied legacy extensions continue to function, because Mozilla is updating them as they make changes to the internals of Firefox. So, I imagine maintaining such a legacy extension is now as much work as it is to just directly contribute and maintain the feature in Firefox's codebase.
[0]: "extensions.legacy.enabled" in about:config. This won't be shipped in Stable, though, and will likely be taken out from Firefox Nightly soon or with the release of Firefox 57.
The important part is ability to replace (overlay) chrome resources, patching/extending the browser code. I hope that's going to stay for a while.
Now there is no requirement to keep them stable even across the patch-level releases. Whenever things would break faster or at the same pace is unknown.