You can place little square graphics yourself that link to the themes directly. I made a few different color modified themes for a client an added something like this to his header
Code:
<div style="float:right;">
<a href="http://linktostyle.com/?styleid=3"><img src="/skin/blue.png" border="0"></a>
<a href="http://linktostyle.com/?styleid=4"><img src="/skin/red.png" border="0"></a>
<a href="http://linktostyle.com/?styleid=5"><img src="/skin/green.png" border="0"></a>
</div>
But i agree, having it all automated would be swell.