From left to right, top row: Win 10, Win 10 + eGPU (using variant of nando4's solution), MacOS Sierra 10.12.3. Attached is a screen of my boot loader screen.
In any case the rEFInd setup may well be of use to some people in a similar boat to me. Caveat: I am on the road right now and don't have my eGPU with me, so I can't tell if the solution actually works. After a few days of intensive research, trial and error, I've managed to get rEFInd working with with a variation of the script posted by nando4.
