In addition to redirecting then when they try to access the homepage, you can also redirect them once they have logged in. To do this, open the Login module in the Module Manager and you will see an option on the left called Login Redirection Page.
For your main question though, this can't be achieved using htaccess, so instead you'll need to use PHP. At first I thought a menu item would do the trick but this will only work if they click on the menu item and not if they manually type in the URL.
So what I would do is, open you index.php for your template and add in the following:
$app = JFactory::getApplication();
$menu = $app->getMenu();
$user = JFactory::getUser();
if( !$user->guest )
if ($menu->getActive() == $menu->getDefault())
Note: I quickly mocked this code up but haven't yet tested it, so let me know if it works or not.
Hope this helps