This should be a space well-suited to promote community and work. You need computers with whatever tools will be needed for what you want to happen. (I'm not sure what those specifics are...at least minecraft, and maybe a Java IDE for mods?) Some people may bring their own tech, so have a place where they can setup and not be ostracized from the rest of the group. If at all possible, it's best to have a setup that doesn't restrict movement (i.e. rows of desks make it hard for those on the inside to get out, this can breed a bit of irritation, which you don't want).
You also needs space away from computers for socializing, eating, etc, as these can be just as important. If you can provide food/drinks/coffee/tea that is always appreciated, but either way choose a location with food places within easy walking distance. Don't forget about parking (free and close are better than far and/or paid).
Reliable internet and tech support are also important considerations.
For a "jam" to be successful, you need to create a sense of community between participants. This means they need to feel united towards a common goal. Just saying "make something" isn't enough, it needs to have restrictions. But, not so restricted that participants aren't able to do something that interests them. Most jams I've done have theme or some sort of prompt, and sometimes "achievements" in addition to the theme.
Depending on the size, you may need to support multiple teams. Having more than 5-6 people working on the same project can be difficult, and you end up with people who do all the work, and people who do nothing (and often neither of these are happy). So, whatever your goal is, it needs to be something that either has multiple parts or allows for different solutions.
It can be difficult to keep virtual participants feeling like they're part of the community, especially if there's only one physical location. Some jams encourage satellite locations (physical locations separate from the main one) so that they can create their own sub community within the larger one. Beyond that, communication is key. Keeping virtual participants up on what's going on in the physical site and knowing their contributions matter is important. I'd probably plan to live-stream the whole event, as well as have an open chat channel somewhere (or in several locations). Someone needs to be regularly monitoring these chat channels, so the virtual participants don't feel like they're disconnected from the main group.
***** I didn't know how much info you wanted, so I'm just going with the basics. Maybe this is stuff you already know, or have thought of, or is irrelevant to your specific implementation. I wasn't able to attend much of the UnSymposium (I did watch a few of the broadcasts though), so I figured I'd toss my thoughts out here.