[titzer]

Yes, if you use bytecode rewriting then all the offsets are changed and you need a mapping. This is one of the advantages of engine-side instrumentation; bytecode offsets don't change. It'll be some time before we can get engines to agree on a standard interface for instrumentation, but there have been some discussions.

Whamm can inject arbitrary instrumentation logic, so you could, e.g. inject calls to imports that are implemented in JS. You'll have some heavy lifting to do on the JS side.