Trapdoor, I have done butt welds in tubular structures where I used a radiused mini to do the corners.  With engineering approval and an understanding with the fabricator that anything found to be in a corner was rejectable reguardless of length.  The fabricator came up with a way of welding these butt welds where he welded thru the corners instead of starting and stopping in the corners.  The flats of the shapes were welded vertical and the welders continued thru the corner and onto the flat.  It isnt easy maintaining rod position as you make the transition but with practice it can be done.  Pipe welders do it all the time.  I considered the ut exam to be a "best effort" and so noted it on my reports.
(CYA).  I hope this was helpful
Dave