> 1) Is it really actual for now?
It's a feature of StAX that Xerces doesn't exist yet support if that's what you're asking. We've been gradually getting help from the community on building a StAX implementation. All of the serialization parts still need work.
> 2) As I understand such XMLStreamWriter implementation could be well localized (in a couple or little more classes), because I just should Implement a class, matching  specification. Of course, there may be some helper classes. Am I right with it?
At the very least there needs to be an XMLOutputFactory (the thing that creates XMLStreamWriters) and an XMLStreamWriter implementation, so that's two classes, plus any other helper classes you need to support a StAX serializer.
> 3) I have downloaded Xerces and Xalan sources and take a short look on them. But I haven't really understood about existing serializers. Is it about something under org.apache.xml.serializer namespace? Should I use something from there or should just use provided Writer or OutputStream?
Yes, org.apache.xml.serializer.* is the base Xalan serializer. You could use this as the base for the XMLStreamWriter (e.g. SerializerFactory and SerializationHandler) if you choose.
> 4) At the first look there are not a lot of work (this description is really simplified):
There's more than that, for instance:
- the creation and initialization of the serializer
- managing and recycling of resources which have been returned in close()
- handling of flush() when the XMLStreamWriter is in various states
- error handling / reporting; message files
- build.xml updates, e.g. to include a META-INF/services/javax.xml.stream.XMLOutputFactory file
Depending on your interest, the scope of the project could be expanded to include more parts of the StAX serialization API (e.g. XMLEventWriter).