One of the flying clubs at my airfield does this every so often, using a C172. The technique involves wrapping the ashes in a small parachute (approx 4' diameter). Said 'chute is then dropped from the window, and the ashes scatter as the 'chute deploys. Apparently, there is usually some 'residue' in the chute, which is tastefully dealt with when the parachute is recovered. A 'Dropping of articles' (and possibly a Rule 5) Exemption is issued by the CAA.
Worked quite successfully when I saw it done last summer. (RIP Mike)