> what are the rough criteria for this to become something to be maintained under the rust-embedded group?

As I understand it, the Embedded Devices WG doesn't take on ownership of things unless they are important to the *entire* ecosystem and the downsides of having 'one official way of doing things' or offset by the benefits of 'now the ecosystem isn't split in two'.

* Microprocessor Architecture and Run-Time support - we all benefit it we all do it the same way.
* Traits for an SPI bus - we all benefit it we all do it the same way.
* HALs for microcontrollers - there are no clear "correct answers" for any given MCU, and the ecosystem is allowed to evolve freely