Using evtest I was able to determine device mapping /dev/input/event0 -> menu-keys event1 -> audio jack event2 -> track pad event3 -> mouse event4 -> keyboard (raw keystrokes) event5 -> system control (e.g. sleep) event6 -> keyboard events (keycodes)
So binding SQUEAK_KBDEV=/dev/input/event6 SQUEAK_MSDEV=/dev/input/event3 then the Framebuffer display works just fine