I've taken a look at this too, and there's two way to solve this:

• the quick and dirty: Remove final modifier on BeanSerializer#serialize, make new subclass to XmlBeanSerializer (UnwrappingXmlBeanSerializer) which override #serialize (same implementation as UnwrappingBeanSerializer). Will also need to add a plethora of constructor to XmlBeanSerializer and BeanSerializer to enable fluent factories.
• the long clean road: XmlBeanSerializer shouldn't extends BeanSerializer. We should instead have a XmlBeanSerializerBase which extends BeanSerializerBase, and then have both XmlBeanSerializer and UnwrappingXmlBeanSerializer extends that base. This however re-orders the class hierarchy and XmlBeanSerializer would no longer be a sub-class of BeanSerializer, so that might cause problem.

I think the second solution is the way to go. I might try that soon since I might need unwrapping support in Xml.

Unwrapping is challenging thing to support, but if it can be made to work, great.

I thin @pgelinas is probably right; and the reason for existing inheritance hierarchy is due to BeanSerializerBase being introduced via refactoring after XML module was created, and its use not having been retrofitted. So now would be great time to add it, if possible.

Little note: I've quickly hacked with the current class hierarchy and unwrapping is easily supported with the Xml module. Like, really easy: all that was missing is a small class mirroring UnwrappingBeanSerializer and the proper constructor from BeanSerializerBase available all the way down and it works. Given that there wasn't anything complicated with that approach, I'll refactor the class hierarchy in the coming days to properly support that feature. Stay tuned ;)

Awesome -- looking forward to this!