OSG has says a concealed cable is OK if:
Provided with a mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like (Note: the requirement to prevent penetration is difficult to meet)
Would it be acceptable to screw an earthed 3mm mild steel plate (Get some cheap from metal supermarkets) over the wiring in a chase ? Obviously it should be wide enough to protect from a hole drilled at an angle. This might be a convenient solution if only a part of the route needs to be protected.
Also, it seems you could run protected cable anywhere, not just in the safe zones, but it would not be a bad idea to still use the safe zones.
Thanks, Simon.