Hopefully this will help someone down the road...here is my final script that works in batch mode from bridge:
var startColor = app.backgroundColor;
if(startColor.rgb.hexValue != 'FFFFFF'){
var white = new SolidColor();
white.rgb.hexValue = 'FFFFFF';
app.backgroundColor = white;
}
var savedRuler= app.preferences.rulerUnits;
app.displayDialogs = DialogModes.NO;
app.preferences.rulerUnits = Units.PIXELS;
var w = app.activeDocument.width;
var h = app.activeDocument.height;
if(w>h) app.activeDocument.resizeCanvas (w, w, AnchorPosition.MIDDLECENTER);
if(w<h) app.activeDocument.resizeCanvas (h, h, AnchorPosition.MIDDLECENTER);
//if w==h already square
app.preferences.rulerUnits = savedRuler;
if(white != undefined) app.backgroundColor = startColor;