I am not sure what you are getting at.Yeah, you can. I remember watching a Pixar documentary and they explained why this was - and I'm paraphrasing - a motherfucker to do and Pixar again have full control over the camera and can stage the lighting manually on every frame.
You sure can have light coming from invisible nonsensical places but if the aim of RT is have realistic lighting aesthetic then you're wandering into some lighting uncanny valley.
The aim of RT is not "just realism". A cartoon movie can be made with RT. Something like jet set radio future can be made with RT. It is just a way of trying to understand surfaces, colours, or directions off-screen - it can be made as wild, cartoony, done-up, fake, controlled, whatever as an artist wants. Hiding surfaces, subsituting surfaces, whatever.