If I understand correctly, it sounds like Donald wrote a custom wrapper that serves as an adaptor for platform specific IO calls. That is not an unreasonable general approach.
Contemporary platforms seem to be split into x86_64 (Playstation, Xbox, most PC-systems) and AArch64 (Switch, mobile devices, M1 Macs). If pure no_std Rust is used, it might be possible for the same two 32-bit .o object files to be used on all platforms (compatibility mode).
Another note is that I want the option to mix non-inline ASM into projects if need be. As long as all object files expose functions that use the C ABI, it should be possible to mix code written in all sorts of languages into the same binary. I only want to write code in Rust and ASM, but some platforms may assume main() is written in one language or another (C++, Swift, etc.). Furthermore, all sorts of libraries are theoretically useful, but my understanding is that in practice many good libraries have already been written in C and C++.
Finally, I seem to recall that @carols10cents is bot interested in Rust development for Playdate, and has opinions. Any discussion that can be generated on the topic is good.