I think your conclusion that the Rust API doesn't support using mmap'd files as a way to read/write arrow files.
In general, I suspect using mmap in Rust is a bit dicey (aka unsafe) as the normal Rust rules of ownership are hard to apply to chunks of memory that can be (potentially) modified by different processes etc. It is probably fine in most read only cases.
I am curious about your desire to use mmap for reading ipc streams (to understand more if we should be looking into mmap support). As I understand it, the ipc interface is designed for streaming reads/writes, and thus the easy random access of mmap seems less important. Are you concerned about Reader performance?