Wouldn’t it be possible to create an intermediate GPL licensed kernel-module which simply re-exported the relevant exports so that other kernel-modules could use the exports provided by this module instead?

It could have a neat name too, like “gplbridge” or whatever (all associations with License-trolls only partially intended). That way they wouldn’t have remove optimized code for Linux 5+ users.

Dirty, utterly pointless, but it would get the job done, right?

Imagine standing in court explaining to a non-technical judge and jury how this "bridge" transforms the copyrighted work into two fully separated works rather than a single derivative work. Do think it would work?

Personally I think one has a better time arguing that kernel modules simply interface with the kernel rather than creating a derivative work, thus making any module independent from the license of the kernel. The case law around "interfacing" is conflicting, but at least a few cases has gone in favor of the accused.