Auto-translated
Perhaps I've completely stopped understanding you... In short, a script is applied at the beginning of the turn, and the hero's movement points are at their maximum. The game tries to add movement points, but it fails. That's it. Then, the movement points are spent as usual. There is no "temporary reserve" or "bonus" given. That's how I understand it.
Here's an idea I had. If the mod has an extra ID for a spell and compatibility with other mods that require that ID is not planned, you could make adding movement points an activatable ability. And, to make it logical, instead of adding 500 movement points every day, you could add, for example, 2000 movement points every three days. This would be like a "forced march." Of course, the description of the spell should mention that it should be used at the end of the turn, not the beginning.