Since Google Earth 5.0, the description balloon has handled pretty close to full JavaScript. There's some interesting things you can do with this. In this example, I'm showing a very basic use of JSON using KML balloon templates.
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
<Document>
<Style id="testStyle">
<BalloonStyle>
<text><![CDATA[
<script type="text/javascript">
function loadData(){
var foo = $[foo];
var myObj = foo.myData[1].value;
document.getElementById("example").innerHTML = myObj;
}
</script>
<body onload="loadData()">
<div id="example">test</div>
</body>
]]>
</text>
</BalloonStyle>
</Style>
<Placemark>
<styleUrl>#testStyle</styleUrl>
<ExtendedData>
<Data name="foo"><value>{"myData":[
{"value":"1"},{"value":"50"}]
}</value></Data>
<Data name = "bar"><value>23094</value></Data>
</ExtendedData>
<Point>
<coordinates>0,0</coordinates>
</Point>
</Placemark>
</Document>
</kml>
Balloon templates are designed to allow you to use a single <Style> element for all or a set of your <Placemark;gt; elements, and then just insert <Feature> specific data into the template. Named <Data> elements inside a <ExtendedData> element provide name/value pairs. However, this can be a little flat. You can put JSON elements inside the <value> element (a child of <Data>, which gives you a lot more flexibility.
The good folks at Secorra provide ObsKML, an ocean observations format, using this technique.
