Why not to use name spaces? An ID reference would then look like id=myNameSpace.actualID

(From Alex: That's just more characters for people to type and more complicated.)

Um... XPath!

var xpath = '//[@scopeid="' + aScopeId + '"]/[@localid="'+ aLocalId +'"]';
var nodes = document.evaluate(xpath, document, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null);
// return (nodes.snapshotLength > 0) ? nodes.snapshotItem(0) : null;

(From Alex: I feel dumb. That's a better solution, though XPath suggests //(scopeidtest)//(localidtest). :) )