Great suggestion about the steam caldera. I could definitely see going that route. The only thing is that I was thinking the east island would contain the volcanic zone, so I'd prefer an eternal storm on the west island.

Could moist winds coming from the north west (as well as the south west), and then hitting a large mountain range in the north west of the western island create such a thing? If so, would that leave the eastern part of the western island too dry?

If the eternal storm gets too hard to justify, I'll go for a caldera. The idea of the eternal storm is just too rad for me to abandon quite yet