The get_themes function passes the available themes to the theme list table class and is hardcoded with a call to asort the array, with no filters available anywhere along the way that i can see.
So in short, not possible at present due to an absence of hooks(at least as far as my observations go).
Relevant files:
http://core.trac.wordpress.org/browser/tags/3.2.1/wp-admin/includes/theme.php
http://core.trac.wordpress.org/browser/tags/3.2.1/wp-includes/theme.php
http://core.trac.wordpress.org/browser/tags/3.2.1/wp-admin/includes/class-wp-themes-list-table.php