You are overthinking it.
How do you make a windows share, where you share some directory of your choosing and give it a share name of your choosing? You don't go through any B.S. of creating softlinks or mounts or any cr*p like that, do you? No you don't. You do it all via Window's GUI.
Do it that same way on Tomato. The only part you can't do is select the directory in the GUI. Instead, you have to specify the pathname as a text string.
Just for grins, I just shared out a deeply nested directory on my router's USB disk, and Windows sees it as \\router\logfiles.
Also just shared out an even deeper nested directory containing music. Windows sees it as \\router\mp3 and is playing one of the mp3's right now.