Try this: set the source of the drop down to the SharePoint Data Source. Click on the XPath button next to the Entries field, then on Filter Data. Click Add, Then select The Expression. Add the following expression, inserting your field and attribute names as indicated: not(. <= ../preceding-sibling::<insertSharePointFormLibraryName>/@<insertAttributeNameForFormNumber>) and not(. <= ../following-sibling::<insertSharePointFormLibraryName>/@<insertAttributeNameForFormNumber>)
This should filter all of the items out of the drop down and leave you with only the max value.
That's all I can think of without writing a little code.