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