Apart from cost - what's preventing this from working with something like the Echo-Express pro? http://www.sonnettech.com/product/echoexpresschassis.html - are there software limitations that will prevent a graphics-card from working in that chassis?
Bandwidth. We can (and already have) gotten video cards working across external links, but they are severely bandwidth constrained.
Thunderbolt is constrained to 10Gbit (20Gbit in some contrived used cases if your'e using Thunderbolt2). A 16-lane PCI-e gaming video card consumes anywhere between 32Gbit and 126Gbit (with modern cards coming in on the high end of that).
So you can get it working, but it'd be mostly pointless. The main impetus for an external video card is to get desktop-like video performance for demanding applications like gaming, but you are heavily bus-bottlenecked which reduces the video performance to a small fraction of its potential.
