Yeah, not sure I'd recommend doing something like that to such an instrument. They may have been entry-level archtops for their time, but...y'know...it's a vintage Gibson!
FWIW, here's a page showing some pics of an ES125 undergoing a truss rod replacement. Looks like a standard Gibson design. The truss rod sits in a curved channel wedged in by a wooden cap along the length of the channel. The point at which the truss rod is closest to the back of the neck would be in the middle rather than the nut:
http://www.mumfordguitars.com.au/3.html
http://s45.photobucket.com/user/shakarocks/media/Trussrodcurveillustration.jpg.html