Well, I was thinking of doing this with Linux, since that's what I use. I'd have Windows be the decoy install and use LUKS for dm-crypt to encrypt the Linux install at the system level. And setting up the default boot with no countdown in GRUB is trivial. All the pieces are there, and while there's no definitive guide on how to do it, I don't reckon it would be very difficult. Since I don't use Macs, I have no idea whether this approach would be feasible for them as well.

The main thing holding me back is that now that I've moved to an SSD, my laptop only 64 GB of space, so I'd rather not waste 10 GB on a decoy Windows install. Perhaps once I get a bigger SSD.