qriouslad on "[Plugin: jonradio Multiple Themes] Auto append ?keyword=value in subsequent pages clicked"

Additional note... potentially related?
http://stackoverflow.com/questions/7266107/best-way-to-determine-whether-a-webpage-is-viewed-in-an-iframe
http://stackoverflow.com/questions/6662542/check-if-site-is-inside-iframe
http://www.daniweb.com/web-development/php/threads/430477/detect-if-page-is-being-loaded-inside-iframe

Source URL in iframe containing the right GET parameter would initiate the PHP function (you will have provide) to set a cookie with the alternate theme, and your plugin would do the work to change the theme. Conversely, sans the GET parameter, the PHP function would delete the cookie and ths, your plugin will revert the WP site to the default theme.