Pretty sure Mac’s won’t support the multi display MST for displayport – the easy way to do this would be to use a MST display port splitter out of a thunderbolt hub or one thunderbolt port but it won’t work with a Mac.
You’ll get screens to mirror, but not act independently. There was talk of adding MST support in Yosemite but seems it’s only added 4K not not the dual monitor support. Thunderbolt itself can’t be split, only chained.
The only way you can do what you’re trying to achieve is as you said above, use the belkin dock (or similar) to give you your gbethernet and display, and use the other tb port for your other display. Doesn’t give you 1 plug ease though. If you’re going to do that you may as well just have a regular usb 3.0 hub for peripherals and ethernet and keep the tb for displays as you’re plugging in 3 cables anyway!
Have you seen this though? http://zenboxx.com/
Not available yet, but if it gets to market mid next year it will do what you want, the retina version will give you gb ethernet and display port from the hub with a pass through for your second monitor, all with 1 plug simplicity. £100 though for a product you can’t have for 6 months…
Maybe one of the CNC wizards on here could CNC up a aluminium block to take all the wires to a dock so it’s one plug????