How does it work to call C for the missing peripherals? For instance using the exiting vendors libraries. The book in question does not seem to cover this, unfortunately.

I think for a long time we will have to accept that Rust and C are going to co-exist on these devices. So that story should be workable.

My embedded code tends to have a strong split between application logic (in platform-independent, data-driven functional code, with automated tests) and underlying hardware-dependent "app host" (as little custom code as possible), with a data-based interface. I would be quite happy doing C for the hardware layer and Rust for the application logic layer.