A small update on this.
I have been working on a project using Zephyr on the FRDM-K64F development board. The project will entail multiple UARTs, SPI for an 802.14.5 radio, I2C for FRAM, RTC, ATEC608A Crypto chip, and an AT24MAC602.
I chose Zephyr (after trying at least 5 other platforms)...