Thanks Dave and Joyce
So my take-away from this advice is that Al wire-Au pad wedge bond is certainly not a definite No-No, but:
(a) the finish must use gold of the right purity and hardness
(b) the nickel coverage of the copper must be very good
(c) the surface roughness of the pads must be controlled
As to whether the reliability of this is at least as good as Al wire-ENIG pad, I guess some aging and testing would be needed?