These days, with nationalism and populism rampant across the world, I think we need a solution where no one country (or country's leader) can simply decide to turn off critical infrastructure for the rest of the world and/or hold the rest of the world to ransom. Then you run into questions of "do we really want (insert bad country) to be able to expose IOT source code to their evil hackers?".
This is a really difficult problem to solve, but ultimately I think ownership of the "keys" to unlock escrowed code needs to reside with (winging it here...) a body such as IEEE or ISO. Or possibly something like a global council where e.g. any 5 countries out of 7 can collaborate via a sharing of keys to release source code, but no one country is able to do so.
Could have the code run in a sandbox where people can apply “external” network traffic trying to hack it (or apply vulnerabilities), inspired by how you can run ML models on kaggle.org on Kaggle servers to validate models.
Have end-points as honney-pots, so if you can access these endpoint you prove you have compromised the code.
If there is no new code with patches the keys are released.
This way FCC/gov don’t need to maintain a technical system. Just build this once.