These are ALL great ideas! Yes, my girls go absolutely crazy for rice hulls. They will even choose scratching through rice hulls over ranging!I am not an expert by any means, so if someone else chimes in with something that sounds better - go for it.
This, however, is my thoughts based on my experience and a bit of judgement (? correct? incorrect?) on my part.
1) If you can place 'baffles' in the coop/run so not all hens pecking the ground with butts in the air will be visible to Scarlett may help. ( something like physical visual barriers like large stumps, hanging cloths randomly within the coop (and/or run), etc. Any physical/visual things to break up lines of sight.
2) Whenever possible, hang some food item for them to peck at (multiple ones at a time) Such as: apples, cabbages ( decent sized ones can be halved or quartered), creating multiple different things to be pecked at, so all aren't at the same one jockeying for access This will keep them occupied with something other than each other's butts!
3) If possible, have hubby move things around in the coop/run once a week. Keep them off balance and ';re-exploring' their area to investigate changes.
4) If you can find any, see if hubby can on the weekend, rake up a bunch of leaves (damp ones are super) and throw them into the run/coop floor. Give them something new to scratch/hunt in/through.
5) Give them a new 'toy' that has treats they have to work for. I don't have a picture of one I have used...but I will try to explain it. Take a ROUND/Cylindrical plastic seasoning container (such as an empty oregano or italian seasoning container) Drill a half dozen small holes in it at various spots around the container. Fill with scratch grain, toss into run. The chickens will have to move/roll the container to get it to 'spill' its contents. Makes them work for the scratch and keeps them occupied. Just make sure the drilled holes are only slightly bigger than the diameter of the scratch grains, so just a little falls out with each movement of the container, and note that you may need to lightly sand around the drill holes to remove the sharp burrs caused by drilling.
Obviously - with #5, don't overdo the amount of scratch they get daily - but a couple of these 'toys/treat dispensers' can give them quite a bit of entertainment with not a whole lot of scratch. It keeps them physically 'busy', mentally occupied, and most definitely focused on something else than their companions fluffy derrière!
6) are you able to easily get a bag of rice hulls? @micstrachan buys them sometimes - her girls love scratching through - I suspect they find pieces of rice mixed in with the hulls. The hulls make a good additive to their bedding, and rice fragments give them incentive to be digging, scratching and pecking through the hulls.
Lastly, if you can purchase a different bedding than usual, just once...and top-off or clean out the coop/run and use the different bedding, it will - for a short time - create interest.
(If the girls bottoms are pretty exposed, then dabbing some anti-peck on them to darken the bright pink of flesh may also help to keep it from escalating. Sorry, I don't remember the name of the product, but there is an anti-peck spray/daub on treatment just for this purpose. - maybe someone else can chime in with the name of the product?)
So, no one of these alone will fix the issue, I suspect, but more space and a couple of these suggestions might make a noticeable difference.
And, while I hate to suggest this, it could be a 'last resort' kind of thing - especially if it is pretty much JUST Scarlett who is the culprit. You might be able to put her in a 'see but not touch' cage/pen within the coop/run so she can't peck the other's bottoms/feathers - at least during the day, for a short period of time to see if it helps. if you do this, please do give her something to occupy her time, so she doesn't develop other bad coping behaviors.
Sorry for the long windedness of this...and know that I am quite sure you have thought of at least some of these things already - but it is all I've got.
Edited to add: @BY Bob has an excellent point that I completely overlooked: level of protein in their diet. Maybe try a feather fixer feed in a separate feeder for a bit to see if that makes a difference? (or just use it in their regular feeder in place of what you are using)feather fixer is usually slightly higher in protein (or, alternatively, pick up a bag of grower feed in your usual brand - that is also usually higher in protein than the standard layer feed.) They can then choose the higher protein if they so want.
I fill my days here watching my chooks on the camera, and at times yelling at them during the bedtime drama - the disembodied voice 
After all she has spent all this time!
)
