sorry I'm not answering your question directly. Let me suggest an alternative. There is a new, completely rewritten OAI webapp in DSpace 3.x, which means when you upgrade, you'll lose these changes to the old webapp anyway. This new webapp is also available as an addon for DSpace 1.8.x, which you're running. You can download the addon here: http://www.lyncode.com/dspace/addons/xoai/
In this new OAI, it's very easy to do such change by simply changing an XSL template:
(In the 1.8 version of XOAI, it's in a different location but it works the same.)
Just add the top level element you want under line 21 of that stylesheet.
Here's an example how QDC metadata looks in this new OAI interface:
Here's more information about the new OAI webapp: