I would look in the source where the name is applied to the context. I would expect that you could set it there. But, the default is java:/ and it's just a name that's all.
I don't have a working example to hand to show you but what you are asking for is for a reference to the datasource to be bound to the component namespace of your web application.
You need to add a resource-ref element to the web.xml of your web application and configure the reference as appropriate.