Well most the spice should come from yeast and temperature control.
But that dose not answer the question. This is what I would do:
Make a tea with some water and some allspice berries. Crush a few and boil for two minutes in water to simulate a late addition. Let this cool then sip a little, then a big gulp then sip and swirl and spit. This should give you an idea of what the process will add to a beer.
You could also buy a clean example of triple, maybe something accessible like New Belgium. Chill the bottle (to retain CO2). Get your capper ready and some bottle caps. Add a crushed berry or two to a bottle, recap and let set warm for a week. Then chill this down and sample. This will give you an approximation of what you should expect from a "dry" addition of the berries.
If you go with the dry idea and you are worried about sanitation, just crush your addition and let set in a tine splash of starsan solution. Or a soak in some vodka for a day or two.
There are other techniques, you could make a tincture or extract, but that is getting complicated.
Sorry I don't have better suggestions or actual reference info, but I have never done this. So all I can offer are my techniques for experimenting with additions.
I wouldn't grind the spice, but rather crush them coarsely between two hard surfaces. That is what I do for Allspice when I cook with it.