A few observations.
1) will your uno be placed so that it can reach all the sensors via bluetooth?
2) why not just add bluetooth to your central server and avoid the time synch wakeup altogether, since the central server will be on all the time.
3) dead reckoning using the timer will eventually drift. You will want them to have your peer to peer network sync on each wakeup cycle and coordinate on the next sleep.
The arduino sleep details are here, (but I am not sure if the RFDUino will work the same way):
you would probably be looking at waking up with a watchdog timer, since there are no specific events other than time triggering your readings.
Good overview of that here:
I think you can eliminate the uno from your architecture, and conserve battery life using hardware shutdown. Make sure your central server has bluetooth and can see at least one of the units. I would peer to peer the solution so that all the sensors aggregate each others information. That way as long as all the RFDuino's can see at least one other RFDuino, they can pass their information forward.
In that design, an RFDuino would wake up, and post its reading to any and every neighbor that it sees, then read any and every neighbor for the values that the neighbor has collected until all the expected readings are present (you would need to preconfigure the network so that you know how many peers to expect). Once each sensor has collected all the information from all the others, you could send a sync signal and shutdown. That avoids sensors shutting down if they do not have all the readings from everyone else, and the sync will ensure that hopefully the sensors do not all stay on too long. So if a sensor wakes up early, it will wait till all the others have sent their data to the network before shutting down. If it wakes up late, the other sensors will all be waiting for it. The sync signal should ensure that the next sleep cycle starts relatively close for everyone, and the system is somewhat self correcting.
In this design, If one of your sensors disappears due to battery life/failure, all the other sensors will stay awake waiting for it. So you would need some emergency code that kicks in after a sensor has stayed on waiting on a peer on missing sensor data for longer than two normal intervals. Then remove that peer from the wait list and continue operation.
Sounds like a fun project, let us know how it turns out.