My opinion is that it wouldn't hurt anything to start working on it now, but you probably shouldn't expect any amazing results until after she is spayed and her hormones have time to die down.
I mean it's not like litter box training a rabbit is usually that intense of a process. All I had to do to litter box train my fixed rabbits was to put hay in their litter box and if they had any accidents I'd clean the poo off the floor and drop it in the box, and whip up any urine. Not a big deal. It's not really an intense training process like it can be with, say, a dog.
The only way to find out what she'll do is to try it and see. If she's really bad and makes a huge mess then it's not a big deal to put the litter back in the cage and wait to try again.
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