Welcome to the Summoners Fate forums! So sorry for the belayed response, I've been super focused getting the website services ready for our public prerelease. Were you a player in our super-early access back when we were still testing with Hero Mages assets?
To answer your question, yes, we are solving the walls exactly like you describe: Tiles of the matching map area will surround the map with a darker shade to indicate it is a non-playable area. It works pretty effective, allowing the game to support various aspect ratios. You can see the effect in our screenshots and videos, like this:
Also, given your rather distinctive name, I've created two possible avatars you can use, one for our Dark Pegasus Unicorn and the other for our Horse Demon. You can change avatar in your forum account settings, take your pick