The problem I see with it in PbP is that decision making takes a long, long, long time to happen in this format. For an overly open setting/sandbox like kingmaker, it could take a week to decide which hex to explore. Then after a few months of that, people have already forgotten what happened in the first few ones, or what they were looking for. I've found in PbP, that there needs to be a bit of a railroad, too much sandboxiness can lead to paralysis. Too much railroad is no fun either, but although this is one of the coolest adventures I've seen, having like 26 or so hexes to get through plus all the bases/dungeons, or whatever you want to call them to explore it just may be too difficult to run.