Cafu Engine

Does CaLight already support refraction? I di not find anything on the site..

At this time, no.

Given the low-frequency nature of lighting in CaLight, and the small angles involved with refraction (i.e. changing the direction in which light rays propagate as a result of refraction), it also doesn't seem very useful?

I thought about realism including underwater caustics and refraction in models with "Glass" Texture... That's why I asked... So I myself think it would be useful in some cases.

I would think you would do those with a shader and projected textures

I agree with Scott

I thought about realism including underwater caustics and refraction in models with "Glass" Texture.

Usually caustics are just a kind top down projected animated texture (as this requires a fast response real calculation is out of the question)
Some texture generators can produce animated caustic textures ...
usually this would be combined with a slope based transition so it fades off on steep angles (to prevent odd distortions)

For refraction of glass and other transparent surfaces you need access to the framebuffer \ render to texture function in order to distort\manipulate the current view.

CaLight is only for static light maps and can not provide any information that requires a constant view-dependent update.