As wiki is lacking more advanced stuff, I am curious if it's even possible to randomize textures inside of ctm/ctm_horizontal/ctm_vertical, as well as, for example, bottom texture and top texture for ctm_vertical?

Not currently. The eventual plan is to allow combining render types, so you could specify random AND ctm together, and apply both effects. There's no reason this wouldn't work, it's just a decent amount of effort to set up the API for it.