Indeed, this is my favorite thread as well.
I had no idea you guys were teamed up with VOA. I know that Florida station very well, as I hear it all the time from California. Granted AM transmitters are expensive, and is the main reason why they are not used as much anymore as a broadcast medium, the delivery of data could be cheaper if the data was distributed. Maybe because I am excited to hear that there is an opportunity for shortwave radio to have a place in a modern application, that I am biased towards utilizing it, but I have to agree with @KR0SIV, that SW radio would be a great way to complement the entire Outernet network.
A lot of Hams I communicate with use packet radio, ALE, WSPR, JT65, PSK31, RTTY, and various FSK modes to communicate, and these would be great modes to use as an exchange. In other words, these modes, since they can't handle lots of data in a short amount of time, would be great for simply supplying a user's receiver with a status of the network. If propagation does not allow for optimal conditions, supply the receiver with satellite information to link up. If conditions are good enough to go over SW, have the receiver link up to several SW stations. This group is doing a lot of work already in this area http://robust-packet.net/ and the Robust-Packet is a lot faster than HF packet.
What is Outernet's goal with having a single ground-based transmission point? To me that seems that we'd be limiting ourselves to a single source of transmission, and that's not what the mission is, right? Ground-based transmissions would not need to derive from a single source, and would not require anywhere near 100kW output. I've worked QRP (low power voice and data transmitting) in Ham radio, and you would be surprised what even 2.5 watts could do. If you planted a small unmanned station in each US state running at 100 watts, supplied by a small solar or wind power source, you could easily get data anywhere within North America over HF. Using an AM radio stations infrastructure from a single point with that much power is unreasonable.
Satellite sources are just as important to a well engineered network, but let's not depend too much on one thing, like you said. Obviously, UHF, L and Ku bands can deliver more bandwidth over a single source, but going with the theme of not having to depend on a single tech infrastructure is the smart way to go, and utilizing as many technologies as possible to perform one task or various ones would definitely be a benefit to Outernet's vision.
Thanks, for sharing that information.