In our new version of the Instant Brand Page you have the option of using a collection as the source for your brands list.
This provides the opportunity to present a very specific index. For example only brands tagged as Mens or Womens or Kids.
It does this by extracting the complete list of the product vendors from the products in the collection.
So by setting up a smart collection for a specific product category set you can then use it for your brand page.
To filter - simply click the following option in either the Page, Slider or A-Z content block and choose a collection.
Do note that each content block works independently so you will need to set the data source for the A-Z index and the page to set them to the same brands list.