I am aware that the Custom Template builder doesn't itself have a direct method to insert that code. You'd have to insert it yourself. I'd been doing some experiments with new arrangements of thumbnails, so I'm very aware of the code methods. It's not impossible, just a little more difficult than average.
I'm puzzled why they wouldn't offer that option for your custom template? Either I'm missing something or not understanding the problem.

Danno
Edit: Thinking on the problem.....I'll guess they are seeing the varying number of pictures as the limitation, i.e. it's not a fixed coding effort (in their thinking).