As I told: You can easily create such a shader with SMEE2. This is a fixed function shader and will even run on very old systems.

But beware: A shader with 6 textures and shadowmap uses 7 rendering passes with 6 tilable textures and several blending textures. This uses much ressources and FPS. Better you create levels for forest, winter, beach and so on.

Gamestudio is not Gamebryo (the Oblivion engine) and even in Gamebryo there is no standard terrain. The terrain system from Bethesda is hand made (I once evaluated Gamebryo).

And I think even in Oblivion a standard cell is restricted to a certain amount of textures. You will probably not find a beach texture in the ice region of the game. They simply load a new cell with new textures when you come to a new region. But they have dynamic loading and so it is no problem.