It's possible to place a link inside of the infowindow of a marker on the map and open it in a lightbox (version => 2.12h):

<a href='javascript:void();'  onclick='javascript:MOOdalBox.open(\"http://www.google.nl\", \"Map\", \"600px 600px\", null);return false;' title=link Title>link</a><br />Example

Replace <url> with your link. If you need a space in the linkTitle use "& nbsp;"

{mosmap text='<a href='javascript:void();' onclick='javascript:MOOdalBox.open(\"http://www.google.nl\", \"Map\", \"600px 600px\", null);return false;' title=link Title>link</a><br />Goto Google'}

JavaScript must be enabled in order for you to use Google Maps.
However, it seems JavaScript is either disabled or not supported by your browser.
To view Google Maps, enable JavaScript by changing your browser options, and then try again.

If you place a link inside the infowindow then it will always open in a new window or tab with Google kml renderer!
You can use a kml-file and GeoXML to open a link in the same window (see kml sidebar).

It's also possible in a kml file with GeoXML renderer (Google kml renderer removes javascript). Add to the description of a Placemark:

<description><![CDATA[<a href='javascript:void();'  onclick='javascript:MOOdalBox.open("http://www.google.nl", "Map", "600px 600px", null);return false;' title=link Title>Lightbox link</a><br />Example]]></description>

{mosmap kml='http://tech.reumer.net/images/lightbox.kml'|kmlrenderer='geoxml'|kmllightbox='1'}

JavaScript must be enabled in order for you to use Google Maps.
However, it seems JavaScript is either disabled or not supported by your browser.
To view Google Maps, enable JavaScript by changing your browser options, and then try again.