I can't comment specifically on the DeCoaster frame, Sean (Sidds) would know for sure. However I've built literally dozens of mk1 and mk2 Hesse and never had to use a 13/16" post. What generally happens is the area around the seat tube clamp and top triangular plate weld takes a beating over the frame's lifetime and it needs a bit of filing to open it back out. Don't be tempted to force it, the mk1 had a tendancy to crack in this area if it was forced.
Get the frame somewhere warm for a day like next to a radiator, don't mess with it in the cold. Get a half round file and run it gently down the seat post tube at see if there are any burrs where the welds are at the top tube and triangle and file them gently. Then run some energy cloth and finally grease down it and check the seat post as well. If that doesn't work then get a micrometer and check the setpost o/d (outside diameter) as I have had a few that were too big due to being rechromed or poor manufacturing tolerance control. If so, but a slightly smaller one
Hope that helps
