I haven't done it yet but I have a 92 Trek 950 (all rigid) that I wouldn't hesitate to load down for a bikepacking trip. You may to check out the "show us your vintage mountain bike" thread in the forum. There are many examples/photos of folks using older mountain bikes for this purpose - mostly all rigid but some with front suspension. They are considered some of the most versatile bikes out there and they can still be had for very reasonable prices.