What are regulations for pyrotechnics on planes?
I'll answer for the FAA since thats the jurisdiction I know. Like most things some may consider this careless or reckless operation as per 91.13 which is the FAA's catch-all for odd occurrences that are not worth explicitly regulating.


  Sec. 91.13
  
  Careless or reckless operation.
  
  (a) Aircraft operations for the purpose of air navigation. No person
  may operate an aircraft in a careless or reckless manner so as to
  endanger the life or property of another. 
  
  (b) Aircraft operations
  other than for the purpose of air navigation. No person may operate an
  aircraft, other than for the purpose of air navigation, on any part of
  the surface of an airport used by aircraft for air commerce (including
  areas used by those aircraft for receiving or discharging persons or
  cargo), in a careless or reckless manner so as to endanger the life or
  property of another.


I cant find anything that says "you cant tie fireworks to your plane and set them off" but that is an unlikely thing to bother regulating. 

The TSA does not allow fireworks to be carried on aircraft and this AOPA article would make it seem as though they are not allowed to be transported by air at all. I would think this includes mounting them to the aircraft so you may be out as a result of this. Your local FSDO may be able to provide some more info on this and what you may need to do to make this ok. 

The other thing to consider is that the FAA may consider this "dropping" so you would need to make sure you conform to 91.15.  


  Sec. 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.