usually in the large offices that i work in the aircon systems have outdoor units that are either bolted to walls or standing on plastic feet, and the pipework is run in risers and false ceilings, and covered with insulation that is slid over the pipe (surrounding it completely)as it is put together and brazed, and would not be affected by fault currents from the buildings other supplies and are mostly out of normal reach.Then that is ok not to bond as i interpret the regs
It would not be practical to bond these back to the met each time as some are on the roof, some the ground, and in different sides of the buildings, as they are generally afterthoughts and not installed when the buildings were new.They are not the same as incoming services such as gas and water that are run in the ground.