As you wrote, what Go has is not really inheritance, the method that allows inheritance like features is called Embedding.
http://golang.org/doc/effective_go.html#embedding
What it means basically is that the embedded struct has no idea that it is embedded, so you cannot override anything that is called from it. You can actually take the embedded struct and take a reference for it only from the embedding struct.
So your best way to do it is more or less like your second example – through some sort of dependency injection using interfaces. i.e – A has a reference to some interface that does the actual work, say worker, that writes to a file or whatever. Then when instantiating B, you also replace A’s worker with another worker (you can do it even without embedding A of course). The A just does something like myWorker.Work() without caring what worker it is.