This is really useful to test new features before they land in master and are deployed out - we actually have tooling that lets you specify a branch you want to try out, and you can quickly have your client pull built assets from that branch - and even share a link that someone can click on to test out their client on a certain build. Just the other week, we shipped some non-obvious bug, and I was able to bisect the troubled commit using our build override stuff.
The worker stuff is completely integrated into our CI pipeline, and on a usual day, we're pushing a bunch of worker updates to our different release channels. The backing-store for all assets and pre-rendered content sits in a multi-regional GCS bucket that the worker signs + proxies + caches requests to.
We build the worker's JS using webpack, and our CI toolchain snapshots each worker deploy, and allows us to roll back the worker to any point in time with ease.