Using 2010, same method. Mine just ignores the Filter and sets the value to all Fields.
<xsf:assignmentAction targetField="xdXDocument:GetDOM("Allegations")/Batch/Method/Field[@Name = "Disposition"]" expression="d:Disposition"></xsf:assignmentAction>
Here is the XML:
<?xml version="1.0" encoding="UTF-8"?>
<Batch OnError="Continue">
<Method ID="1" Cmd="Update">
<Field Name="Title"></Field>
<Field Name="Disposition"></Field>
<Field Name="MCI"></Field>
<Field Name="ID"></Field>
</Method>
</Batch>