We'd never get as far as allowing you to fly it, though. Our books say that we should expect you to fly it, precisely so that we know what might happen, so that we go in and positively control things.
Is it not the case that you would get a 0DME 27L at some stage if you were overflying 27R, because DME south has been zeroed to read 0DME at both the TDZ of 27L and the TDZ of 09R. That arc would continue to intersect 27R at some point.
I switched a BA 319 last week, at 3 miles. He was wings level on final at about 400ft.