Google could publish the code they're going to use to do the transformation step (as e.g. a Docker image), and the developer could audit the result by optionall

Google could publish the code they're going to use to do the transformation step (as e.g. a Docker image), and the developer could audit the result by optionally independently running the same code themselves, at any time after publication. As long as the code Google published was deterministic, the developer should be able to use it to reproduce a binary with the same the SHA as the one offered centrally. This is the same process that package maintainers (e.g. Debian, Mozilla, Tor) go through to produce reproducible (i.e. verifiable, deterministic) binary packages: they enable arbitrary users to run a copy of the packaging + distribution infrastructure on their own machine, such that they can watch it all go and see that it spits out the same thing the remote server does.