The only way that you can do this is to have a reference to a grouping node passed into the script. This means we need to have it included in the VRML script definition. The URL system is very similar to that used to create VRML content from a string. THe major difference is that instead of returning a node and adding that to the scene you actually get the information back through an event. That event must be an MFNode eventIn.
Lets look at our problem again.
This is the group that you will be adding the box to. The first is an array of strings. This array is a list of URLs that can be retrieved. Like the rest of the nodes that use URL strings, this refers to the urls in decreasing order of preference. This must be a pre-existing, legal VRML node reference.