Ah, sorry. (:+:) is not an Applicative. It's still possible to make instance Monoid:

instance Semigroup (f a) => Semigroup ((f :+: g) a) where

InL x <> InL y = InL (x <> y)

InL _ <> y = y

x <> _ = x

instance Monoid (f a) => Monoid ((f :+: g) a) where

mempty = InL mempty