every company i've seen only has bolt-on necks and bodies
Never seen a kit. Most places would figure if you have the skills and tools for this kind of project wont need them to put all the parts in a box to. Or wont want the basic setup.
a kit is suposed to be more affordable. So they wont use neck-thru guitar since bolt-on is cheaper.
Carvin sells neck-thru blanks if that helps, but I've never seen a kit.
Stew Mac has pre-made neck-thru necks. Altho they only have 25.5" scale. I think thats as far as your gonna get, as making the body wings and doing other stuff is just (generally) basic woodworking skills.