You have provide very good example of Freertos for LPC55S69 what is run in nonsecure world and task can call secure
But it is not so clear if if privided example can be transformed to case when RTOS running in secure world
and can have nonsecure tasks as it mentioned here:
Case 1 – RTOS in Secure state When a real-time operating system is running in Secure state, threads can be Secure or Nonsecure. It is also possible to have cross-domain API calls between Secure and Non-secure software, which means each thread can have both Secure and Non-secure stack allocation.
Does this case is covered in that port of RTOS what is part of SDK by using minor configuration/modification
OR it require quite massive implementation ?