I'm not positive but I might be visualizing your weldment.
Would you be able to go to a groove on the open side of the 2nd 7/8" plate? You might not need to provide a full penetration weld, just a deep enough groove and a reinforcing fillet to get adequate throat?
Can you pre-fabricate a "spool" or stub pipe section, with the ring and 7/8" plates on a bench, and then insert and weld the spool in?
The only other thing I can think of I have never done but have often wondered about. Could you modify your FCAW gun with an "extended" contact tip? I was thinking that some copper tubing could be threaded to replace the contact tip and put in a slight bend at the end for the electrode angle. You could insulate the tube and then reach inside the joint. Possibly, some 0.035" wire would be flexible enough feed through without kinking.
Kind of a "Rube Goldberg" approach but maybe it would work?
Chet Guilford