I don't know of any PV output optos that have 10V kind of outputs, which
is what you're going to need for Vgs on a big FET.
An isolated supply is fairly easy - as in an off the shelf DC/DC
isolated converter. You could use it as the gate drive directly, since
you're not looking for high speed.
For improving off isolation, you could put multiple identical switches in
series. I would start by simply taking the device and measuring its off state,
to see how many I might need to chain up.
Or a configuration that has a series switch followed by a shunt to ground.
Here's one that will do 10V (it's far from the only one). These parts are designed for turning on MOSFET gates to I suspect they stack as many LEDs in series as they need to get the required voltage.
Besides being cheaper than an isolated DC/DC converter, they probably have lower capacitance across the isolation barrier.
Eamon
