Excellent work, I think this is a fabulous idea and should be the de facto standard for these locked devices. Were you still thinking about using a 2nd data/cache partition as well?
I think it would be pretty sweet if you could loopback mount file system images stored on internal storage for the system/data/cache partitions to implement a proper and safe multiboot system. I don't know how much of a performance hit this would take though.
Anyway, the one suggestion I would make about backing up the original system is to not do it as part of the regular backup/restore. Since system should rarely change, it'd be a waste of time and storage space to back it up every time. Could you either make it a separate option, or perhaps only back it up if the partition's checksum changed?
Besides, if you hose your system and need to restore it, you're probably not going to be using this recovery anyway.