The second hack for making proportional symbols in KML, is to use regular polygons (see the first hack here and here). Unfortunately, KML has no build-in support for regular polygons so you have to do it the hard way: by calculating the longitude/latitude for each vertex or corner point of the polygon. Since I'm not a math genius, I was happy do find this PHP function by TJ which I could modify for this purpose. This is the result:
By changing the bearing or angle 30 times from 0 to 360, the function returns a circle (KMZ).
Reduce the number of iterations to 4, and you'll get a square (KMZ). Not the easiest way to draw a square, but it works.
These files are also viewable in Google Maps and Microsoft Live Maps (Virtual Earth), even though the circles are not so regular any more.