Petter,
This should get what you're looking for (things in bold you'll need to change to match your implementation):
var dataGridDij = dijit.byId(dojo.query("[id$='GridServerSideId']\")[0].id);
/*
* Using Firebug or Chrome's developer tools:
* The console log statements below will allow you to see all the currently selected documents
* and their properties that you can access via client side javascript.
*
* The 2nd console log statement should show you all the properties of the data grid dijit itself
*/
console.log(dataGridDij.selection.getSelected());
console.log(dataGridDij);
var unid = dataGridDij.selection.getSelected()[0].attributes['@unid'];
window.open("http://yourserver.com/path/to/db.nsf/yourXpage.xsp?documentId=" + unid + "&action=editDocument");
I highly recommend that you check out the Dojo API for the data grid. I'm sure you'll find many interesting nuggets of information there that should help you find the properties that control or publish whatever you may be looking for. You can find that documentation at :
http://dojotoolkit.org/api/1.4/dojox/grid/DataGrid. In order to set any properties that are part of the data grid but not exposed via the Extension Library component you can just add them as a dojo attribute to the extension library component.
HTH