I just bought a Asat Custom Classic Semi-Hollow that has a pretty full size neck. There was a Asat Classic hanging next to the one I bought that was even bigger. Certainly a lot larger and rounder than I remember G&L necks being. Over all this is a great sounding and playing guitar.
The earlier G&L's had thinner necks than most of the more recent guitars. If you want the beefier neck, then stick with the guitars made after 2000. You can order a G&L with a oversize neck. I played one that had the same profile as the Fender Nocaster. 1.0" depth along the entire neck. I loved it. The owner wasn't interested in selling it. They do show up occasionally on EBay.
As far as my experience w/ them is you can only change the width,radius,frets or type of wood(maple, birdsyemaple). Never heard that they offered an oversized neck like the other boutique companies(Suhr,Anderson,etc.).
my G&L Z-3 came with a large chunky, perfect V-neck... as perfect as a big Tele (not a Nocaster profile tho') and my R8 and McInturff Soft V in their own ways. I've never seen a Nocaster neck on a G&L but there are some big ones out there.
the neck on my ASAT classic bluesboy is VERY thick. I don't have the dimensions on it but it's a bat - way fatter than my MIM 50's Classic Tele was and that's a fairly chunky neck. I didn't custom order it so that's just what it came with - they're out there.