I guess I'm having a bad brain day. Help me, Obi-Wan.
What I want to do is add a custom-field-derived element, let's call it $customstuff, before the_content for every post. $customstuff will be part (or all) of the excerpt, then at the top of the full post.
Where do I put it? I mean, I know something goes in functions.php for my theme, but how do I structure it? Do I have to repeat function thematic_content() but stick in my $customstuff code, and if so where?