When you select a commander in Brawl with the keyword Specialize (PC), even if you have the alternate card art favorited, it defaults back to the basic design. From there, even if you drag the alternate art to the commander slot (or hit Apply Style), nothing happens.

I'm assuming it has something to do with the way the client handles picking your commander's second color on selection, and given the mechanic's age and niche situation I doubt WOTC will give this a second look, but I'm hopeful. Just spent a good chunk of gold on a card art I can't apparently use.