Why is that an issue? Presumably firmware updates are at best a monthly occurrence. Why do I care if it takes two days to push? Just make sure you have reliable resume and error checking.
While this seems simple, most mesh network protocols use different routing protocols, network topography will be unique for every network, and even the nodes are ephemeral (especially if battery / mobile). All this means that your data throughput is highly unpredictable.
Assuming that you have a border router (or similar persistent member of the network) who is in charge of disseminating your firmware over the mesh, you can either go with an algorithm that is unicast or multicast.
Unicast can be tricky because you don't really want different firmwares running on the same network (especially if the firmware includes a networking stack update). It also takes a long time, because you have to send X bytes N times.
Multicast can easily flood a network, even with rate limiting, because the amount of hops or rebroadcasts required can result in exponential growth in network activity.
Simply extending the amount of time it takes is problematic. What if a device restarts? What if it drops off the mesh?You are definitely correct in that it is solvable. It is not incredibly simple, though.
