Could someone make an extension to re-enable this config option?

And... would Mozilla agree to sign it?

(Since, you know, the standard response over the years to users complaining about removed functionality in Firefox has been, "use an extension")

no, because then any malware would immediately install that extension first, and then flip the about:config bit on the filesystem, and then install their extension, and then you're right back where you were before signing.

I doubt an extension could do that since the signing mechanism needs to be secure, otherwise it would be pointless.