Is it allowed to scatter human cremains (ashes) from an aircraft?
AOPA has an entire article on it that's worth reading and while it's generally fine with the FAA (assuming the pilot isn't being careless or reckless), there may be other regulations or laws that apply. The article mentions:


Permission needed for scattering over federal land
Permission needed for scattering over water (federal and local regulations)
State burial requirements


But there are two specific aviation regulations that might apply to the flight. First, 14 CFR 91.15:


  §91.15   Dropping objects.
  
  No pilot in command of a civil aircraft may allow any object to be
  dropped from that aircraft in flight that creates a hazard to persons
  or property. However, this section does not prohibit the dropping of
  any object if reasonable precautions are taken to avoid injury or
  damage to persons or property.


Second, you said that you plan to have a "contract" with someone to scatter the ashes, which implies that you're going to pay the pilot. If that's the case, then the pilot may not use an experimental aircraft for the flight. That's because the FAA considers carrying human ashes to be carrying property (Harris interpretation, 2009) and 91.319(a) doesn't allow carrying property for compensation in experimental aircraft:


  (a) No person may operate an aircraft that has an experimental
  certificate—
  
  (1) For other than the purpose for which the certificate was issued;
  or
  
  (2) Carrying persons or property for compensation or hire.


Finally, note that accidents can and do happen and in at least one case two people died while spreading ashes (although to be fair, the toxicology data was fairly 'interesting' in that one).