There is a hidden choice: Baremetal Once you generate a project for the RT1020, you can delete the framework = ... line in the platformio.ini, then PlatformIO will not compile in any frameworks (of which Zephyr is the only choice for that chip), only the sourcse from src/ and lib/ (and lib_deps).
That way you can provide your own startup file, linker script and all sources to e.g. put in FreeRTOS. An example of baremetal projects is e.g. atmelavr native-blink or my stm32 blinky or this close to baremetal STM32 FreeRTOS project.
For the RT1060, there is also the choice of mbed-os.
There are open issues to implement a builder script for the Arduino core from Seed that would make these chips usable with Arduino.
If you want it to support also other Frameworks / SDKs, like NXPs MCUXpresso SDK, open another issue there. Mbed-OS for one board, Zephyr for all, or baremetal for all are the only current choices.