I don't know about the X-T2, but in the X-T20 manual there's a documented "Cable channel cover for DC coupler" -- see part #36. http://fujifilm-dsc.com/en/manual/x-t20/about_this_camera/parts/index.html
It's a rectangular section on the right edge of the battery cover.
I know on the X-T1, you had to have the battery grip to use the DC coupler. In reading about the X-T2 battery grip, it sounds like you'd simply use that instead of the separate power unit. From what I can tell, if you have the charging cord plugged into the grip and then turn on the X-T2, the camera is powered but the grip isn't charged.
It sounds like you have been very ingenious in what you have done. I doubt that there would be any official way to do it. I wonder if the grip, which gives you a total of three batteries, would do the job. Expensive but not invasive.