I've yet to build it but I have plans to use 1/4" hardware cloth shaped to funnel the manure into a 5 gal bucket. the bucket would be sheltered under the hutch so as not to collect rain. Urine goes through, berries roll to bucket.
But, it sounds as though your rabbits are not on a screen floor. So the task is more one of separating bedding from the manure. A straw bedding might be more useful for you than others. If using straw like material then 1/2" hardware cloth would be useful to make a classifier (fancy word for sieve). Berries fall through while straw remains on the screen.
The problem with pellets is that they are not uniform in size and when wetted with urine, they fall apart. That makes it hard to separate berries since either direction in size will yield a mix of manure and bedding.
I remember this one brand I think it was called guardian that you activated it with water and then POOF there was fluffy sawdust bedding in 30minutes. I think it's going to be a " let's try this...." I like the bucket idea and funnel
Around here, the bunny berries just fall to the ground under the hutch where they pretty much dry out without any effort on our part. When it's time to add them to the garden, they're just scooped out from under the hutches. Since the buns here are angoras, they don't get bedding since it would get stuck in their wool so unless they're on wire, the system here may not be of any use to you.