[Creator of the StarlingMonkey JS runtime here]

> It works in the browser already, by bundling another browser runtime engine into wasm.

Note that that's not how JCO works: JCO unbundles a Component, emitting core wasm modules plus JS glue code. JCO also is a toolchain that can orchestrate turning JS into a Component, and that does bundle a JS runtime (StarlingMonkey), but that's not for running Components in JS/browsers, but for running JS in Component runtimes, such as Wasmtime.