Hi J.,
Could you post a screenshot of the UI, so we get a better idea of what you're talking about? And is this a form you created with Form Builder, or are you writing the XForms "by hand"? What is the datatable you are referring to; is it an fr:datatable?
Maybe you can react to an xxforms-index-changed on the repeat that generates the table and upon receiving that event do an xf:setvalue that sets the values of the drop-down.
http://wiki.orbeon.com/forms/doc/developer-guide/xforms-events#TOC-xxforms-index-changedAlex
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