Can the wiring be run over the top of the ceiling joist, or must it be run through bored holes when running perpendicular to the joists?
This is a non-habitable space, will have an attic entrance only for the purpose of blown-in insulation and is not tall enough to stand in. There will be no junction boxes, lights or any other reason to enter this space.
Best Answer
Accessible or not
While you might call this area inaccessible because it's small, dark, and difficult to enter. In this case, the codes definition of accessible is whether or not there's permanent stairs or a ladder.
Cable run across the top of joists
If the attic is accessible by permanent stairs or ladder, any cable running across the top of joists will have to be protected. Protecting the cable is as simple as installing "substantial guard strips", which can simply be a 1x2 furring strip on each side of the cable.
If there's no permanent stairs or ladder, this protection only has to be provided to cables within 6 ft. of the entrance of the attic.
Cable run through bored holes
You could always run the cable(s) through holes bored in sanw lumber joists, but you'll have to make sure the edge of the hole is more than 1 1/4" from the nearest edge (2" according to IRC).
NOTE: When dealing with engineered structural members, boring holes may not be an option.