Contiki is not really a full OS.

Its some embededded device firmware with a text mode UI added.

No multitasking is possible and even no program main loops because apps need to poll the event kernel regularly.

For apple IIe I recommed writing something like GeckOS/A65 that supports preemptive multitasking.

Even A/UX 0.7 can do more than contiki ,but that needs a PMMU.

Get something with multitasking +TCP/IP +sockets + multiple FS support ported to mac and add a real GUI.