As it turns out, all of Mendonca's vocals are exhaled. I found a place on the Akercocke forum where he said that.

I think you're right about them being some sort of gutteral.

Speaking of gutterals, I sure am having trouble doing those. I hold my throat as open as I can, and I keep my mouth in the "O" shape, but it always comes out sounding like a low growl instead of a gutteral. You know, with a little bit of gurgle and rasp instead of that smooth, airy gutteral sound.

Apparently I'm just not grasping the concept of how to do them I guess.