Consulting the documentation on spawn restrictions, this is the generic form:

{
"type": "ez_supervisor:distance_below_sea_level",
"distance": 5
}

Wanting to add a restriction that a mob can only spawn from a spawner, I would want to use ez_supervisor:spawn_type:

{
"type": "ez_supervisor:spawn_type",
}

Checking the docs, the additional parameter would be "type":

{
"type": "ez_supervisor:spawn_type",
"type": "spawner"
}

but since this is JSON, I'd expect those two "type" keys to collide. Checking the code, it does look like "type" is expected:

Basically, if I've got the big stupid right now, I'd request an example on using spawn_type. If not, maybe "type"->"reason" or something.

Mod's been fantastic otherwise :]