Using a shim to do a legal workaround seems to me as a trick that is unlikely to ever get past a judge. In all other form which people try to do legal workaround in order to turn illegal to legal, it seems that the law don't generally take kindly to it and either explicitly forbids it or just treat the trick as inconsequential to the end result.
For example, we have shims in form of dummy organizations, straw owners, money launderer and so on. Neither of those kind of shims work to turn something illegal to legal. Why would some dummy code that sits between two incompatible software work any better in the legal system?
"Neither of those kind of shims work to turn something illegal to legal."
There's nothing illegal here to try to turn legal, though. If the blob's license permits other software to interface with it without restriction, then the shim is free to be under the GPL per the terms of the other work from which it's derived (Linux).
The only way there'd be anything illegal here would be if the blob itself is a derivative work of Linux, and if that's the case, then the blob itself is illegal (since it would be subject to the GPL), regardless of the shim and regardless of whether or not it's distributed as part of a Linux distribution.
Take a dummy organization that is used to avoid taxes. There is nothing illegal to pay someone to form a company located in a different country. A company can also sell assets to an other for what ever price they want, and there is nothing illegal to declare that as a result one of the companies now have zero profits and zero assets.
But if something is legal or illegal depend more than just on each set piece. If combining a blob with the kernel creates a derivative, then just adding a shim to the mix won't turn it legal. Judges are trained to look at the big picture rather than the individual parts, and this is especially true in civil-law. Court systems are generally interested in:
What is the kernel authors intention by their copyright license.
What is the blob authors intention by their copyright license.
What is the distributor intention when combining the two works and what understanding did the distributor have about the other copyright authors and their wishes in regard to derivatives.
The existence of a shim that has no other purpose is a sign that something shady is going on, similar to dummy organizations. The question a judge is likely to ask is why such obfuscation was used since that can help establishing intention. If the end result is a de-facto derivative, and the intention was to create a single work out of two seperate works, and the creator knows that they are not allowed to create a derivative work, then a shim is not going to save the day.
"Take a dummy organization that is used to avoid taxes."
See, that right there is where the analogy falls apart. Tax evasion is (usually) illegal. Writing software to translate between two independently-developed programs is not, last I checked.
"If the end result is a de-facto derivative"
You'd need to prove the blob is derived from Linux. If it's not, then there's literally nothing illegal happening here. If it is, then - again - the shim is not the illegal thing; the blob is, and the shim is entirely irrelevant in that illegality.
The usual reason for the shims, by the way, has very little to do with licensing terms (at least not directly) and very much to do with the fact that Linux does not provide a stable API (let alone ABI) for kernel modules. The intent of the shim is therefore almost always technical rather than legal in nature.
For example, we have shims in form of dummy organizations, straw owners, money launderer and so on. Neither of those kind of shims work to turn something illegal to legal. Why would some dummy code that sits between two incompatible software work any better in the legal system?