Hi Christian,
You can use the xxf:itemset() XPath function for this. For instance, if you have a control named 'a', you can get its items with xxf:itemset('a-control', 'xml')/itemset/choices/item. (Note the '-control' after the control name, as this function takes a control id, and this is how Form Builder generates controls ids from the name you entered.) And for more on the format of the data returned by xxf:itemset(), see:
http://wiki.orbeon.com/forms/doc/developer-guide/xforms-xpath-functions#TOC-xxf:itemset-Alex
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