Which is exactly what I did. For GN, I used "PBM Go board number ####" and for the file name, I used "BlackPlayer-WhitePlayer-BoardNumber-YYYYMMDD" and Richard has already put all of this into place, except for the most recent changes, which added the game start and stop times and fixed an oversight that had caused the board to be correct from left to right, but upside down. I have submitted those changes, but they have not yet been put into place.Do you intend to define a new property "ID"? That would not be recognized by SGF-reading Go game display programs, which presumably is the reason why you are going to all this trouble to create sgf files in the first place.
There already is the property GN. Here's the description from the SGF website: