Don't rule out her litterbox training yet. We've had 8-10 year olds learn how to use a box!
Here's what we do:
1.) get her spayed -- this is important. Especially because she's older, she needs to be spayed and see if she's got uterine/reproductive cancer. Sometimes you can get lucky and catch it before it spreads. (If she's already spayed, ignore this. ;PP)
2.) Put her into a large cage -- even if your other bunnies are ok using a smaller cage as a 'crate' and get most of their time out, this won't help her learn the litterbox right now. She needs to have the ability to get clearly out of the box and away from it right now to help her learn.
3.) Put several LARGE litterboxes in the cage (say, one in each corner). Large like an under-bed container or a storage tub with a door cut out. When she makes poop accidents, move them into the box. When she makes peeing accidents, clean them with vinegar and then put a litterbox there. You may end up with a cage FULL of litterboxes, but they do eventually get the idea. Make sure that there are at least two (rabbits seem to like having a place to lounge in the box and a place to eat) and that at least one has hay IN it.
When she's reliably using the boxes, start taking them away, one by one. I'd still leave two, as we have a fair amount of bunnies who just like having two boxes and will appropriate a corner if there aren't two.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry