Brilliant, as always. Thank you sir.

My final code ...
For those looking for a split menu solution that shows a submenu or, if none, another module to fill the space, here is the final code I used where module 49 is the split menu module with display level starting at level 2.  The level 1 menu item being displayed becomes the Split Menu's title.  Module 59 is another module to fill the space when there are no sub-menu items to show in the split menu.  Module 60 is another module to fill the space when the page being displayed is accessed through another menu / the main menu is inactive.  (Remove "} else { return 59;" and "} else { return 60;" if you don't want anything displayed other than the split menu.)  Change 'menu-main' to the value you see for "menu type" when viewing the list of menus in the Menu Manager.

$menu = &JSite::getMenu();

$active = $menu->getActive();

if ($active->menutype == 'menu-main') {

if (@$active->level > 1) {

      $topParent = $menu->getItem( $active->tree[0] );

    $changes->mod(49)

      ->title($topParent->title . " - Sub-Menu:");

    return 49;  // item in submenu active

} else {

  $items = $menu->getItems(array("parent_id"), array($Itemid));

  if (count($items) > 0) {

    $changes->mod(49)

      ->title($menu_genius->info( "title" ) . " - Sub-Menu:");

    return 49; // active level1 item has submenu

  } else {

  return 60; // active level1 has no submenu

} } } else {

return 59; // main menu not active

}

I love how easy it is to do this with MMP!

I needed to supplement this as I was getting an error for pages selected from  non-menu modules as it tries to render the sub-menu for an inactive menu item.

The error was
Notice: Trying to get property of non-object ... in mod_metamod/helper.php(597) : eval()'d code on line 3

So I added @ to suppress the error on line 3, yet the result is the mod not rendering at all instead of the else clause (return 59) being rendered.
Line 3

if (@$active->menutype == 'menu-main') {

I tried a wrapper code to do a preliminary test for non-object to trap the eval failure and show a substitute module.

I added

if (!is_object($active)) { return 58; }

but, curiously, it was not enough - mod didn't render even though Debug advanced said Includes 58.

So instead I'm detecting whether the item displayed is from com_content.
Current code:

if ($option == 'com_content') { 

$menu = &JSite::getMenu();

$active = $menu->getActive();

if (!is_object($active)) { return 58; } // no active menu object

if ($active->menutype == 'menu-main') {

if (@$active->level > 1) {

      $topParent = $menu->getItem( $active->tree[0] );

    $changes->mod(49)

      ->title($topParent->title . " - Sub-Menu:");

    return 49;  // item in submenu active

} else {

  $items = $menu->getItems(array("parent_id"), array($Itemid));

  if (count($items) > 0) {

    $changes->mod(49)

      ->title($menu_genius->info( "title" ) . " - Sub-Menu:");

    return 49; // active level1 item has submenu

  } else {

  return 60; // active level1 has no submenu

} } } else {

return 57; // active menu not main menu

} } else {

return 56; // main menu not active - not com-content

} 

and at last, this renders 56 instead of not rendering.  Not sure why it worked and is_object didn't.

The page link triggering failure is following a link to full page event view in a JEvents module listing coming events, but I suspect following in-module links from photo gallery random images and similar will cause the same issue.