Can the ECS work in an unpressurized cabin? Can you control the cabin temperature if the cabin is unpressurized? What if the "cabin" isn't really that big, like a fighter aircraft?

For example, could I cruise in comfort at 20,000 ft in an unpressurized cockpit with an oxygen mask and heater?
If the cabin is unpressurised, the temperature could be controlled to a limited extent- this is usually done in a number of helicopters. 

You can certainly cruise at 20k feet without a pressurized cabin - WWII pilots regularly did this with supplemental oxygen and heated clothing- though I'd hardly call it comfortable.