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), because it came with a full IPV4/IPV6 network stack (client/server, DHCP, SNTP, PING, IPVv6 autoconfig), supported OpenThread (which is what I am going to do with the radio), and contains built in USB modules (including CDC-ECM which I want).
I have done enough experimenting to know that all of those things are going to work under Zephyr. So I believe I have committed. Zephyr certainly takes some time to understand, and can be a configuration nightmare until you are comfortable with it.
My same application can be compiled to run on the FRDM-K64, FRDM-K41Z, NRF52840, and now:
Teensy 3.5 and Teensy 3.6.
Zephyr uses the concept of 'Boards' that define the MCU, the connected pins, and what device drivers are required. For the MK6x series, it uses the HAL from the MCUXpresso SDK (it looks like v1, but could be v2)..in any event, you really don't need to worry about that. Just that I haven't found a module on the MK6x that zephyr doesn't support.
I decided to move away from the FRDM-K64F, because of it's limiting ability to connect I/O, with only an Arduino style connector with fixed pin placements.
Moving to Teensy was a no brainer, completely configurable but lacking the I/O to attach to. So I designed a 'carrier' board that would allow me to add all of the additional hardware I wanted/needed. I am calling it 'Teensy Explorer'.
The board includes a slot to plug in the Teensy, 4 Mikro-Bus 'Click' sockets (hundreds of devices), a socket for the upcoming ETH PHY board, optional power inputs and regulation, outputs for all 5 UARTs. You can 'stuff' the board with whatever features you need, with the bare minimum being the sockets for the Teensy, and a couple of sockets for click boards.
I should have the new board back from OshPark tomorrow (I already know of a couple of bugs), and I will post updates...
...but Zephyr 2.2 is running on both Teensy 3.5/3.6 models.