So, I did that, and the resulting 404 page was hopelessly borked.
Luckily, it turned out to be much, much easier than it had to be. All I had to do was be stuck at work for a while waiting for another team to do something before I can begin my work. This gave me some "free time."
Looking in the index.php file, it would appear the 404 is built in. So I copied just the top part of the file, and the bottom footer call. I added my own little flair in the middle, and here is my result:
list($bfa_ata, $cols, $left_col, $left_col2, $right_col, $right_col2, $bfa_ata['h_blogtitle'], $bfa_ata['h_posttitle']) = bfa_get_options();
<h1><?php _e('Not Found','atahualpa'); ?></h1>
<p><?php _e("Well that's embarrassing! I can't seem to find that!","atahualpa"); ?></p>
<p><?php _e("You could try one of two things. You can either:","atahualpa"); ?></p>
<h4><?php _e("Search","atahualpa"); ?></h4>
<p><?php get_search_form( $echo ); ?></p>
<h4><?php _e("OR","atahualpa"); ?></h4>
<p><?php _e("You could try the old site, <a href='link-to-old-site'>link-to-old-site</a>.","atahualpa"); ?></p>
<?php get_footer(); ?>
I saved that as 404.php, and bingo! Everything works as expected.
Now I will use the same template to create a custom author.php file.