It's my understanding that currently the only Mac hardware that specifically supports an outboard 5K display (like this one) is the Mac Pro and that this only came along with the recent 10.10.3 update.
If one were to get an outboard Thunderbolt PCI expansion chassis, and a video card capable of driving a 5K monitor, and a recent, Thunderbolt-supporting MacBook Pro what are the chances that such a machine could successfully drive such a monitor?
It seems like it should be doable, but I admit that it'd be a LOT of "moving parts" to expect to all work together just right...