Welcome Sam!
Some good posts here so far.
I agree with the idea of selecting individuals from the shrubs and trees already growing there, then encouraging them. In that respect, it might be tempting to remove or prune-back some of the ones you don't select, to make the 'hedge' more distinct as such. I would advise caution in doing so, as some of those other plants might be contributing to the microecology (climate, lighting and soil) that the 'selected' plants need or prefer.
Some other things to consider...
Yes - digging holes for plants in tough soil/rock can take an age, but fence posts also need holes.
However, if you do put in fencing or at least fence posts with straining wires, you might be able to cover a greater length of 'hedge' by using climbers. That is, one planting hole for a climber might provide the same hedge-worth as ten holes for shrubs. Which climbers? Well, Bougainvillea and Senecio angulatus come to mind - trying to choose plants that will look after themselves.
And you need the plants to be self-sufficient in terms of watering, because it is expensive in terms of irrigation piping and water to supply a long hedge. And it could be looked-upon as wasteful of water.
Whatever plants you choose, consider how they will 'fit' in the environment. With your large plot in at least a semi-wild setting, many cultivated plants could look out of place and could prove disruptive in the ecosystem. The Bougainvillea and Senecio I mentioned could be problematical in those respects. But at least the B. should not give problems from seedlings. Wild asparagus will form a dense if straggly hedge (at least it does here in Cyprus) (see topic
here).
As well as selecting from plants already growing wild on-site, look at the wild plants slightly further afield. You might find, eg., figs (Ficus carica) growing wild and with their roots apparently in solid rock. Figs come easily from seed, and if you plant them small (suitably marked so you don't lose them), they stand a good chance of getting their tiny roots into the crevices and becoming self-sufficient and steady against winds - more-so than if you planted more mature ones. Obviously with small plants, you can buy more for the same money, they need much smaller planting holes, etc.
Where the rock is inhibitive to wild plants (let alone planted ones), consider dry stone walling.
