Is it possible to return a specific tag archive/template as one of the search results?

A simple way to do is to edit your theme’s “search.php” (copying it from the parent theme into your child theme’s directory first if you’re using a child theme) and then before the posts loop putting something like:

        <?php
            if ( $tag = get_term_by( 'name', get_search_query(), 'post_tag' ) ) {
            ?>
                <article><!-- or whatever is consistent with rest of listing. -->
                    <h2 class="entry-title"><!-- ditto -->
                        <a href="https://wordpress.stackexchange.com/questions/174104/<?php echo esc_url( get_tag_link( $tag->term_id ) ); ?>" rel="bookmark"><?php printf( __( '%s (tag archive)' ),  $tag->name ); ?></a>
                    </h2>
                </article>
            <?php
            }
        ?>