i worked from the original default TwentyTen - 2010-weaver might have some modification in the code that are no more compatible with the original TwentyTen -
use your idea, seems to work great.
no reason why the changes would have triggered an error (typo?):
the actual changes were only in this part:
sprintf( '<span class="author vcard"><a class="url fn n" href="%1$s" title="%2$s">%3$s</a></span>, date modified: %4$s',
get_author_posts_url( get_the_author_meta( 'ID' ) ),
sprintf( esc_attr__( 'View all posts by %s', 'twentyten' ), get_the_author() ),
get_the_author(),
esc_attr( get_the_modified_date() )
added this: , date modified: %4$s' after </span>
and this: , esc_attr( get_the_modified_date() ) after get_the_author()
(nothing complicated, and worked for TwentyTen )