Charlie.l

SOLVED ! 
The camera is working perfectly with my original coupler snugly sitting in the power grip and running through the regular USB plug. Just like you suggested.
The fact that my X-T1 was powering down the whole time, when directly connected to the dummy battery, must have been because the connection was insecure as I couldn't't close ******  door.
All good- thanks again !

I do not think there is a regular fix of the kind you probably are hoping to read about.
There are/were battery grips folks could install that held an extra battery (Fujifilm’s part number is VG-XT1). Third party versions of the grip are still bring sold.  If you look around you may be able to find one with a battery door cover that works as you are hoping.
p.s. Welcome to the forum.

There's a "cable channel cover for DC coupler" along one short edge of the battery chamber cover. It's item 36 in the Parts of the Camera illustration on page 3 of the manual. I bought a CP-W126 DC coupler, which has an end that looks like the battery and slides into its place. But how do you remove the cable channel cover?
It is obviously a separate part. It's a lighter color than the battery chamber cover, and it's not quite as hard.
I studied it under a magnifier and it seems to have little tabs and ledges that would prevent moving it in any direction. Do you force it? In which direction?
Thanks!

Hey, great answer! It took a little more force than I expected but seeing your photos and reading your description gave me the confidence to go ahead with it. I really appreciate you taking the trouble jerryy!

New and Improved Answer! As intimated earlier, now with images!
 

X-T30 ready for the change.

 
Remove the L-Bracket and the battery.

 
Grab the tab and slide it out.

 
And up.

 
Insert the CP-W126 DC coupler.

 
Close the battery cover and it is ready to go. When you are finished using it, reverse the process.