My mistake, the Data In property is available.
Looks like I can achieve what I need with the following:
dataOut = event.source.parent.getComponent('Row Selector').dataOut
dataIn = event.source.parent.getComponent('Row Selector').dataIn
if dataOut.rowCount==dataIn.rowCount:
filename = "All"
else:
filename = dataOut.getValueAt(0,0)
system.gui.messageBox(filename)