Currently surface guis just align to the box boundaries of a part. This means they don’t work on wedges, meshparts, or unions. Decals do have the desired behaviour but the issue with them is you can’t efficiently use them for customisable text etc.
From the way I imagine surface guis to work, think this behaviour would be very difficult to produce. So maybe an entirely new, alternate object that allows us to write custom texts on parts with surfaces that aren’t flat would be a better solution.