In the OS2 version of the Instant Brand Page there is an option to use a data source besides the default store vendor list to populate your page or A-Z listing.

Simply select an alternative product collection

its recommended to use a smart collection which uses a vendor and/or tag Condition

The Brand Page or A-Z links will now only show the brands for the products within the respective collection.

This is a great option to use when setting up a brands listing for specific brand categories like Mens and Womens