How can I add/append content to the_content on the home page via a plugin?

You mean a filter and a check for is_home()?

add_filter( 'the_content', 'wpse6034_the_content' );
function wpse6034_the_content( $content )
{
    if ( is_home() ) {
        $content .= '<p>Hello World!</p>';
    }
    return $content;
}