It simply has not been implemented as its never been needed. Create a feature request here:
If you have a patch to add the functionality that can be submitted there as well.
While I was poking around NamingContext, I noticed a couple things. First, calls to NamingManager:getObjectInstance pass in the absolute name, rather than the name relative to the context. This seems contrary to the docs (http://java.sun.com/j2se/1.4.2/docs/api/javax/naming/spi/NamingManager.html)
The name and nameCtx parameters may optionally be used to specify the name of the object being created. name is the name of the object, relative to context nameCtx.
Also, the createSubcontext method doesn't replicate the current context's environment on return. Is this intentional?
If these are just oversights, let me know and I'll post a patch; I'm just a little hesitant to go mucking around code I just started looking at.