Official answer: no.

Unofficial, and more importantly, unsupported answer: it should actually be possible to override the built-in terrain shaders with your own: Download the built-in shaders, and add the Terrain splat shaders to your project, and change them as you like. However, we may at some point in the future decide to change how the shaders in the terrain engine are used - this would break any web players trying to replace them in this matter.