Pigeon grit available in the midwest, between Chicago and Milwaukee, is a nice dark red granite, with some oystershell and some senna added for an odor attractive to the pigeons so they will peck and scratch. The oyster shell won't raise the pH a lot, probably not as high a pH 9, but it will raise the pH above 8. Relatively few trees prefer alkaline soils, though many will tolerate alkaline soils. Most trees that are listed as coming from Karst areas have some tolerance of limestone. Certain species of desert habitat juniper prefer an alkaline reaction, while the majority of junipers, including Shimpaku would prefer a mildly acidic to neutral pH, in the 6.5 to 7.0 range. Some trees will simply die in soils containing lime and or oystershell, azaleas, blueberries, andromeda, Kalmia and other bog habitat dwelling trees.
So because of the oyster shell is definitely not to the liking of some trees, lethal even, and the majority of trees are helped by oyster shell, they may be indifferent at best to it. I would not use pigeon grit for any trees unless I knew that particular species required an alkaline soil to do well.
My 2 cents.
So like John Kirby, it is not something I would be likely to use on any of my bonsai.
By the way, because certain orchids I grow are obligate calciphyles I do keep crushed oystershell on hand. It just does not get used on my trees. Only the orchids and a couple cacti.