I have a load of ideas how Tableau could better check the data after a JET import - they are out there in the ideas forum. Another way to do this is to bring them in as text (either with a dummy row of data or a schema.ini), then make the conversion in Tableau.
Thanks for the suggestion Ryan. You can create a schema.ini to tell Tableau to load the file as floating point. I realize that's not the easiest answer, but it will work.