When I say

import Data.Type.Coercion
import Data.Coerce
import Data.Proxy

eta2 :: Coercible (f a) (g a) => Proxy a -> Coercion f g
eta2 _ = Coercion

I get

    Could not coerce from ‘f’ to ‘g’
      because ‘f’ and ‘g’ are different types.
      arising from a use of ‘Coercion’
    from the context (Coercible (f a) (g a))
      bound by the type signature for
                 eta2 :: Coercible (f a) (g a) => Proxy a -> Coercion f g
      at /Users/rae/temp/Roles.hs:5:9-56
    In the expression: Coercion
    In an equation for ‘eta2’: eta2 _ = Coercion

Unlike the case for #9117 (closed), this one is not expressible in Core. (At least, I don't see a way to do it.) So, to do this, we would have to update Core and then update the constraint solver. I do think this would be type safe. But, I also think it's reasonable to wait for someone with a real use case to shout before doing this. The only use case I have is to be able to do this:

-- says that a's parameter is representational
class Rep (a :: k1 -> k2) where
  co :: Coercible x y => Coercible (a x) (a x)

instance Rep f => Rep (Compose f) where ...