Okay, I've set up a Debian stable builder as well... However, Debian stable has a 3.16 kernel, and wireguard-dkms requires >=3.18, so installation fails. Assuming 3.18 is a hard lower-bound on kernel versions, it looks like jessie will never run in-kernel wireguard. Fortunately it'll be moot in a few months when stretch becomes the new stable.