After reading the source, I found out that what I want to do is impossible since form buttons are explicitly NOT read by the dojo.formToObj.

But you CAN do this:

// in your form:

Output Files

//and in your header

function sendFNDdata(pressedBtn) {
     var vargetFNDdata = {
         url:"cgi-bin/PortSim/getFNDdata.pl",
         handleAs: "text",

         // Here's the FM!  
         content: { value: pressedBtn }, //<------!!!!!!!!
         form:dojo.byId("fndParms"),     // yes they can be combined
         
         timeout: 5000, // Time in milliseconds
         load: function(response, ioArgs) {...},
         error: function(response, ioArgs) {...}
    };

 dojo.xhrPost(vargetFNDdata);
};

Also, note that the xhrPost is wrapped in together with everything else into one function; that wasn't clear from the docs (at least to me).

I hope this saves someone else six hours of frustration!

--

faber