Bagheera,
Two problems with that idea: jitter and grazing angle.
Jitter: airborne designators have tracking and stabilisation mechanisms to produce a steady (ish) laser spot. Ground designators don't: the laser goes exactly where the unit is pointed (it's aimed through an eyepiece). Even if you could build some kind of mounting that took out the worst of the helicopter's vibration, it would need continual manual adjustment of aim through the eyepiece to compensate for helicopter movement. At several miles' distance, even the smallest angular jitters multiply up to substantial movement of the laser spot, which reduces accuracy and runs the weapon out of energy as it continually attempts to "correct" its flightpath.
Grazing angle: ideally you need to be shining the laser at 45 degrees or less from the perpendicular to the target surface so that laser energy is reflected towards the incoming bomb and not scattered away from it. You can get away with shallower angles but performance becomes progressively less assured; the 30 degree dive described above would be marginal. To achieve a 45 degree angle from 10,000ft altitude your helicopter would only be 10,000ft horizontally from the target: definitely not safe to sit in a hover! Even reducing the grazing angle to 30 degrees it would need to be 20,000ft away: still vulnerable. The only way it could work at longer ranges would be against targets with a sloping or vertical face against which the laser could be fired, with the weapon approaching from the same direction. At longer ranges jitter would be even more of a problem, as above, and vertical target faces tend to be much smaller than horizontal ones so the chances of keeping the spot on target would be even less.
In sum, not likely to be successful.