Tapering spiral text - tried text on path, art brush, and envelope distort

Using Illustrator CC 2014

I'm trying to create a spiraled text in which the text size decreases as it spirals inward. I've tried putting the text on a path created using the spiral tool and executing a script to decrease the size of each character, but I couldn't figure out how to access my text selection correctly.

This is the kind of spiral I'm looking for.

I've also put the text on a straight path, expanded, and used the free transform tool to make it a tapered art brush. This achieves what I want with straight lines, but the spiral doesn't render the brush in an acceptable way.

Using brush shape on spiral path.

I also tried applying a triangular brush to the spiral path, typing and expanding my text on a straight path, and making an envelope distort using the spiral as the top object. The results were kind of awesome, but not even close to what I was looking for.

I think the best way would be to figure out how to execute a script on the text selection. I already have the basic logic of it solved (using JS):

function ChangeSize(){
    var myDoc = app.activeDocument;

    //Here's where I need to make a proper selection
    var selection = activeDocument.selection;

    //begin resize function
    for (var i=0; i<characters.length; i++){
        characters[i].characterstyle.textheight -= i * .05;

    }
}

This is the only thing holding me up from an otherwise very simple and straightforward design. I've spent several hours trying and I've got nothing. Does anyone have a simple solution for making my spiral look pretty much exactly like the Pi spiral in the sample t-shirt picture?