@Alan : for some (explainable, see below) reason, I keep forgetting that you are going this route, with a PSU and a separate voltage converter unit.
For the prototype Zion PA, I went for the cheapest Chinese 20A PSU I could find (it's a power supply for LEDs operating in CV mode, cost me around $20 shipped) and I would never trust the built-in protection of such a device. I'm not even sure it has any, and if it has it might just be a non-resettable fuse which doesn't really warrant that you won't blow a component or two before it goes away. But you get what you pay for anyways, the unit you got is surely of more acceptable quality.
I'm not sure to understand the part where you say it would prevent from over-heating. Could you please explain by which mechanism it works? Temperature probe in the DC-DC converter?
As for the reason now: I was wondering why go the trouble of having two external components when the Zion guts are probably small enough to be integrated inside the device itself and are already proven and tested for the job? The Murata chip is super small and surprisingly capable!
I know that you came from the Milaana PA you built, and thus followed this path. But I think that it would be better to create a desktop Zion instead. This is why in my mind it was important to have a keyed connector like the XT60 or the Deans, to protect against reverse polarity (which fries instantly the poor chip)
About the mica plate(s), two things: inertia and health concerns.
I did a lot of experiments with various SS-based heaters, different configurations, materials, geometries, even toyed with mesh, and it's clear that the large surface area is the way to go for instant convection. But that doesn't explain why the Zi and Mi have that incredible inertia: you can release the trigger and continue gulping vapor for a long while (several seconds, esp. in the Zion) We like to call that "riding the falling edge of the heat wave"... but I digress!
If you say that the heater mass is quite light in comparison to the insulator plate(s) Could it be possible that they play an important role in that inertia effect by creating some sort of heatsink?
Now you may wonder why would I want to have inertia in the device? To be honest I don't know, apart that I'm just used to it... Thinking of it, I'm not even sure this is a desirable trait. As long as the heater can handle and stay steady under large enough draw speeds... All this energy that went into heating the plates is more a waste than anything... At the same time you called your technology "heat island" for a good reason too, no?
Finally, seeing problems with mica come from breathing its dust and particles, and even if our use scenario is surely very far from creating harmful concentrations... Wouldn't it be better, just for the peace of mind to revert to glass? Deep lung inhalation on a daily basis does constitute a chronic exposition afterall. Sometimes it's the frequency that is more problematic than the dose. But I'm no materials safety expert.