I have seen this done from a multi-engine acft. A small length (~2ft) of fibreglass drain pipe about 3 in in diameter - modify one end to make it easy to insert the ashes or put in a sock with end tied off till you want to activate. Place the pipe out the strm window and undo the sock. Have a pilot in the other seat so you can do the job and not have to fly the acft at the same time. If the pipe is out far enought there should not be any 'blowback'. A practice run would be good. Good luck