Hi Neal,
from what you write, I'd suggest to go for Fiasco.OC as it runs on ARM Cortex and provides real object capabilities. In my opinion, this kernel is not a perfect choice esp. looking at minimality but will get you the best available. Note, for several reasons Genode on OKL4 does only support version 2.1, which disqualifies it in both regards - ARM and object capabilities.
Regards