(1) Since even is monomorphized to Int -> Bool, you can just write

f (Foo n) = even n
f (Bar n) = even n

(2) Since the sum subtypes are the same, you can write

data Foo = Foo {myN :: Int} | Bar {myN :: Int}

f :: Foo -> Bool
f x = even $ myN x

(3) As to your original question why the underscore _ can't be overloaded for this, I dunno. It /may/ be possible. Seems rare in practice. You could always apply the type isomorphism to get

data FooT = Foo | Bar
data Foo = Foo Int FooT

f (Foo n _) = even n