@thealchemist @zkwc @djpaul
Hurray!
The code snippet works just fine in functions.php. That’s good news for a theme designer: no worries about a theme overriding a user’s custom configurations in bp-custom.php.
Now I’m off to add “Group Landing Page” as an option in my new theme. It’ll be incorporated with the theme’s “Alternate Header” option too.
Then I’ve got to adapt the code to work with member profiles. My initial attempts have failed, but I’ve been known to be persistent at times 
For future reference, here’s the code snippet to add to functions.php or, if you have created it, bp-custom.php.
You can see it in action here: http://rspacesandbox.org/ Click any group.
function redirect_groups_to_forum() {
global $bp;
$path = clean_url( $_SERVER['REQUEST_URI'] );
$path = apply_filters( 'bp_uri', $path );
if ( bp_is_group_home() && strpos( $path, $bp->bp_options_nav['groups']['home']['slug'] ) === false )
bp_core_redirect( $path . $bp->bp_options_nav['groups']['forum']['slug'] . '/' );
}
add_action( 'wp', 'redirect_groups_to_forum' );
