I have done it. One of the most moving and memorable flights of my life.
Widow was in the back seat as we circled and scattered his ashes over the particular patch of ocean that he wanted to be left in.
Ashes came in a little plastic box, about 4 inches per side, and I had someone else in the front take off the cap, hold it on backwards, then when fully out the storm window remove the cap.
In a tight turn he let them stream out, then after a while, he dropped the cannister (deliberately). The last of the ashes streamed out of the container as it took the final fall to the water. It was that glassy that widow saw the splash from 4000'!
Do a trial run a few days before.