The lightest I know are Gotoh Stealth, which you can get from Shane at High Mountain. They're pricey though, over $100 per set. I doubt he could get them any cheaper without a huge order, but it never hurts to ask. I wouldn't mind getting a few more sets myself if it does turn out that mid-size orders get a discount.
LMI has Gotoh open backs for as low as $40, which should be reasonably light weight, although I've never tried them myself.
There's also the option of minimizing weight on the headstock itself like this beauty by Bob Matthews http://www.luthiersforum.com/forum/viewtopic.php?p=482119#p482119
My 14 fret mahogany steel string necks with two way truss rod, closed gear tuners, and barrel bolts have been pretty ridiculously heavy, even when fairly slim and narrow. Not to mention the relatively large headblock necessary for the bolt-on tenon. It's annoying, especially after experiencing the wonderfully light weight and perfect balance of that coral snake guitar
I'm switching to lighter tuners for sure, and probably either dovetail or integral neck, and perhaps a change of reinforcement. Could ditch the adjustable rod entirely in favor of one of these carbon fiber D rods http://dragonplate.com/ecart/categories.asp?cID=130, although they're expensive in addition to losing the adjustability. Or maybe just use light weight woods like Spanish cedar or butternut (if I can find any) with carbon fiber stiffeners and a two way truss rod. Or switch to a single compression rod. Also going to try a 12 fret neck on my next one, which will help too.
gotta love the lower weight. I disassembled a maple dresser That I picked up off the hwy to lake perry in lawrence .It yielded a lot of soft maple which I figured is = to h.mahog in wt . less all the hassles of pore filling. I bought an el cheapo $17 harboring freight gram scale which I find extrememly useful. I have an old ohaus scale that I used for bowmaking in the past.. I might just put the michigan maple neck on the back burner.Have not installed the lmi truss rod / bolts, since I got so many necks out of that soft maple. I might switch to that instead for my OM build , your thread here reminded me that once all that hardware plus tuners are put in , it/s gonna be one heavy neck.Just laminating my first maple spanish heel style neck.I built a solera abt 6 yrs ago from the courtnall book . but never used it. Need to dish out a 25 ft radius on the solera. cheers . How is the plane coming along? PS as for neck stock 1/4 sawn sycamore is nice, cheap and readily available from jim in g view.Metro has sp cedar but I personally don/t care for the smell, stringy nature, an tearout of the wood while carving , because of the weight, one needs super sharp tools , an pore filling and I/m geting lazy in my old age 
. Metro had slash sawn butternut , which allegedly is light , but I don/t care for the colour . I really like the peruvian walnut he has , light as h mahog , beautiful colour ,an easy to carve., and IMHO looks stunning.