Take a look at Ribbon Gallery Control by Greg Maxey. I do not believe it changed the icon for a building blocks gallery even though the images used in the signature control are in both AutoText and Custom 1 building blocks galleries. They are also stored separately in the xml. However, it may be an alternative. Both the XML in it and the vba are accessible.

I earlier referred you to Build & Deploy Custom Building Block Galleries by Greg Maxey. I will look at your template as well but do not hold out hope of changing that icon.

Keep in mind the ability to also use the Building Blocks Gallery Content Control in a document or template.