Okay, so I've thought about this more. (Perhaps way too much, actually.) My thoughts are that there are a lot of factors here that need to be considered carefully before we make any firm commitments. Here are the ones I've thought of:
1) Target Audience / Publicity
Simply put, who is this con for? Is it for just the Acheboard crew, or is it intended to be aimed at all Achewood fans, TOUAMB-reading or not? If the intent is to reach out broader than this board, we also have to figure out how to publicize it to a broader Achewood fan audience. This is bound to be problematic unless we can get Onstad on board.
2) Location
This is pretty obviously a big deal; you want a location that is as convenient as possible to the most number of attendees. This means that you can't just pull a location out of your hat and go with it -- you actually need to figure out who is coming from where and find a decent compromise. (This is especially important if it's a small group, since with a sufficiently small group there will usually be obviously more or less convenient locations. If most of the people attending your con live in the southeast, it might be best to shoot for Atlanta over San Francisco.) Don't assume it has to be in a huge city if the attendance will be relatively small; as long as the location has a decent hotel willing to put up with a convention and reasonable amenities (restaurants, transportation hubs, etc.), its size is unimportant.
3) Costs
This is sort of tied up in location, but it's still a big deal. How much is this going to cost people, and is this a cost that your target attendees can pay? This is particularly important with something like a webcomics gathering, when a lot of your target audience is college kids and the like with limited incomes. Think about this.
4) Timing
Obviously, choose a weekend that people are going to be able to get free. This is mostly obvious -- major holiday weekends are probably bad, as are late September dates for college students, and so on.
5) Activities
The crux of the matter: what will actually happen at the convention? Will there be a lot of scheduled events, or is the focus just socialization? Do you have enough stuff to take up the length of the convention? An Achewood con strikes me as particularly problematic in this regard, since there's less intrinsically to do in regards to Achewood than there are other, more active enjoyments. You can't really just sit around and read the strips, you know? Where do you see these "multimedia presentations" coming from, anyway?
6) Guests/Onstad
I sort of have my doubts that Onstad will be willing to come to this sort of thing, given his apparent attitude towards the fans and towards this board in particular. You're right that merchandising might sway him, but I hesitate to bank on a massive merch rush; I don't want it to become an unspoken assumption of the con that everyone will buy a T-shirt. It also strikes me as having much potential for awkwardness.
Thoughts?