While in the Wind River Mountains, I found a bronze plaque that marked the spot where a person's ashes where scattered, and while that was done in the early 1930s, it made me wonder if it would still be considered acceptable today (the spreading of ashes, not the plaque)?
It is, however there are some guidelines to follow, and if it happens inside a national park in the United States, a permit may be required.
The guidelines are,
- The ashes should be scattered and not buried.
- They should be scattered away from water sources and developed areas.
- No marker should be left behind.
- The group involved should take care not to damage the area by trampling the vegetation.
I assume you are not talking about laws here ("can I carry an urn with ashes around"), nor about moral (ash-based, not outdoor-based) issues.
So, Wikipedia gives us 7 principles for Leave No Trace.
plan ahead and prepare
travel and camp on durable surfaces
leave what you find
Those are either not relevant to your undertaking, or are obvious anyway (e.g., when walking to your chosen spot, don't trample the flowers).
That leaves us with...
dispose of waste properly,
For all intents and purposes, I would not consider your ashes "waste"; it is a completely natural product. It is what is left if you burn an animal, they are certainly not toxic to plants they land on, or if swalloed (as individual little particles) by animals. Even if you dump them in a neat heap on the ground, no animal would go and eat big chunks of that. Lastly, if scattered properly, nobody will see them, so it is not a visual problem either.
If you want to make sure, you can ask the company which created them for you whether they used any toxic chemicals in the burning process, but, frankly, I would be very highly surprised if they did (only imagine "yes, we use highly toxic Xyzinine to incinerate your grandma" - not conceivable). I also assume that those corpses are naked when cremated (or at least clothed in some non-plastic overalls or sheets to avoid air pollution or the use of expensive filters).
minimize campfire impacts,
This comes closest. You are basically throwing the remains of a campfire in the air; likely with at least some wind (for reasons of ceremony). Those ashes will spread over a large area, you will not dump them just in a spot anyways (for said ceremonial reasons).
be considerate of other visitors.
This would likely be the main concern. Please don't do it on a lively mountain summit on a sunny weekend midday with lots of tourists around... obviously don't do it where the ashes would land on the property of strangers, and so on.