I just bought a Megalite and, yes, the zipper is full-length.
Warning, though- WM bags tend to be claustrophibic. The Megalite is their "wider-girth" model and I find it very much a tight mummy-style bag. (And while I may not be svelte, I'm not fat, either.) I suspect that I'll also most often use it in quilt mode.
That said, mummy bags that fit closely tend to be warmer, so long as they aren't so tight that the down gets constricted.