Constructing coercions for associated type?

I'm working on a GHC plugin in which I need to programmatically construct a coercion for an associated type, given the type function name and the type argument. For instance, I have the following class:

class Encodable a where type Encode a encode :: a -> Encode a decode :: Encode a -> a

along with a type instance, say `Int -> Bool`. In a GHC plugin, how can I apply a type function to an argument type to get back the resulting type and the associated coercion? (Given the latter, I think the former comes from `coercionKind`). -- Conal