There is no solution for this as of WP 3.5. There are a couple of core tickets about introducing the ability defer the generation of specific images sizes on the fly rather than at upload time to prevent the generation of multiple images that will never be used. However, they are far from being included in core at this point. See http://core.trac.wordpress.org/ticket/15311 and http://core.trac.wordpress.org/ticket/22100.
