Reshiram and Zekrom's potential Formes might also be last-minute additions, with Reshiram switching its Fire type for Dark, and Zekrom - Electric for Light. As I've said before, it would explain the juxtaposition, which is something that needs clarification.
Of course, the Pokémon that tend to be added last are the event-exclusive ones. Will Arceus' counterpart (720 base stat total) have a new type? It would make sense to distinguish it from Arceus that way.
Do you recall at what point in Gold and Silver's pre-release period the then-new types were announced? I wasn't around back then and there are literally no active archives for this kind of information. I suspect that it happened a month or so prior to the games' release, and if so, it is a point worth mentioning in the thread.
I just went down to the birthday page and I was all like "DUDE! IT'S JEFF'S BIRTHDAY! I'm GOING TO CELEBRATE DESPITE HIM BARELY KNOWING ME!!!" And then, I came to your page, and I posted this, and then I partied.