Final post now that the job is done. I opened up the floor and found that there's only a short length of soil pipe beneath the boards, perhaps six inches or so, before the pipe disappears into the mountain of concrete. So no chance of moving the outlet closer to the wall using bends etc and as stated above, no appetite or time for major excavations.
The "back to the wall" toilet is now in its permanent position about eight inches from the wall, and everything is working fine. The flush is thunderous and the drainage system is getting a darned good purging for the first time in years.