Here's a thought: While Mathilde can't remove this limitation, it's possible that Algard can. He's been working on Ulgu-as-dimensional-barrier for at least as long as he's had his Screaming Towers. If I was in his shoes and I saw Mathilde's notes on her new spell, I know I'd be thinking of that, though it's not likely he has the AP free to experiment on it.
Probably not going from this spell since the basic idea here seems to be deploying a lot of Skywalk to basically build a temporary road.
A river crossing spell would probably be something like temporarily connecting both sides of the river by exploiting Uglu's moment of transition shenanigans or blurring the distance between two specific points for a limited amount of time. Very useful but also very different from a moving fogbank-road thing.