If I could contribute to the tread drift and follow up on Bruce Tolley’s comment, in terms of ecological impacts, the question is not simply whether materials will get used by the system and how quickly, but rather is more about how inputs of different chemicals might change the ecology of a system.
Many of the unique places we like to go backpacking have interesting and specialized plant and animal communities that have evolved under particular soil chemistry conditions. These unique soil chemistries are often manifest in deficiencies of certain elements, several of which coffee grinds may contain (particularly nitrogen). As just one example, adding nitrogen to a system can change the carbon to nitrogen ratio of the soil, which in turn can change the competitive interactions between different plant species. In central CA coastal prairie systems, non-native invasive species outcompete natives where the N:C ratio is higher. According to this recent paper (https://www.mdpi.com/2571-8789/2/3/41), soils in mid-elevation, west slope Sierra Nevada have only between 19-24 g of carbon per kg of soil ( ~0.2% by weight). According to a couple internet searched sources, used coffee grinds are about 2% nitrogen by volume. I’m not sure about the biological availability of that nitrogen (my recollection of the N cycle is fuzzy), but it seems reasonable to assume (particularly if we ascribe to the precautionary principle) that it may not take much grinds to change soil chemistry to a degree that might change ecological processes, at least at small spatial scales.
Back when I was making fresh coffee and didn’t pack grinds out, I would broadcast them widely to avoid leaving a concentrated slug of nutrients/chemicals in a single location. However, the fact that coffee grinds are a popular and effective addition to garden-bound compost bins ought to give one pause about dispersing them in the unique, nutrient-limited systems that we get so much enjoyment from visiting.
Oh yeah, +1 on Mount Hagen for nice flavor, decent price and avoiding single serve packaging.