You should be able to use the [DESCRIPTION_AND_IMAGES] tag in creating a custom template. That tag reverts selection of method and placement of images to those used during template selection in the one-page lister.
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. I wonder if there is a licensing or copyright infringement issue, but using Auctiva methods on an Auctiva template build doesn't seem like a problem to me.
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).