Sandboxes are working on OS level, meaning that it applies to how apps and their used storage are treated. That doesn't have anything to do with the ability of flashing (installing) a different OS on a device.
Doing this would require a bootloader that can be altered/hacked. This hasn't been achieved yet, and I highly doubt it will ever be done. The only thing coming close to that is some hack some guys were able to perform on a Samsung ATIV S (or so, not sure about the exact device), but that hack was far from what would've been needed in order to get an other OS working on said device.
Now that the boatloader it's been hacked by windows internals maybe now it's time to flash windows phone and install Linux Ubuntu or anything else but I'm still waiting for the genius to rewrite the drivers of this Lumia phones