just my 2c on the backplanes idea - stick with The KISS Principle 
In other words - don't go that route. IMHO it makes the whole design more complicated that what it could be, adds a bunch of extra fail points and really what's the gain? Saving $3 from a converter chip? Which you'll pay in extra connectors cost and more expensive boards and assembly.
If you really want to do chains - just add the option to use plain cables. Not that many people will go that route - you may as well just provide a few test points on the boards and those willing to experiment can solder the few wires - not a big deal.
In other words - don't go that route. IMHO it makes the whole design more complicated that what it could be, adds a bunch of extra fail points and really what's the gain? Saving $3 from a converter chip? Which you'll pay in extra connectors cost and more expensive boards and assembly.
If you really want to do chains - just add the option to use plain cables. Not that many people will go that route - you may as well just provide a few test points on the boards and those willing to experiment can solder the few wires - not a big deal.