@ArjanS: thanks for the link to trace.me -- they seem to have some really interesting products in this space! I've contacted them for some quotes and it'll be interesting to see what they say.
All: what is involved in making these devices (e.g. the ones from trace.me) work with TTN? Is it reasonable to assume that the devices would come with the option of specifying a prioritized list of networks for which the device should attempt to connect to? Or would they typically only have a single network? I've seen the notion of a network key being used, but I'm fuzzy on whether this applies here or if it's just for communication between gateways, routers, etc. Also, is it fair to expect there to be some kind of fallback option, i.e. try TTN first, then if that doesn't work try the next on the list, etc? I wouldn't be opposed to relying on a commercial network in the event TTN is unavailable, but of course that would be hard if every device required an activation and maybe infeasible if each activation required a substantial subscription fee.