If you are keeping her in a warmer place for a few days then I would suggest you go the whole hog - sit her in a bowl of warm water, and soak her rear end (use a sponge to dribble water over it), rubbing the feathers between your fingers until the solid poop gradually dissolves and falls off. (Add one or two drops of dishwashing liquid to the water if it helps, but no more than that.)
It will take a lot of patience from both you and your girl, but it will be worth it.
Once you have cleaned her as best you can (or until she starts to complain too much), dab her with a towel, and dry her off with a hairdryer. Start the hairdryer at a good distance away from her, bring it up to her gradually, and she should be ok with it. Even my most nervous hens don't panic with a hairdryer if I do it in the evening when they are starting to roost. You won't get her feathers dry to the skin, so you need to keep her in the warm place for at least 24 hours to let her down feathers dry out completely.
If the build up is too solid to clean off with water then you need to consider cutting off the tops of the feathers. Any significant amount of poop left on the feathers will cause more poop to stick to it and cause a build-up. Cut only the essential 'messy bits' off, and she should be ok. A small amount of feather loss won't cause her too much harm. (Chickens do moult over the winter, after all!)
Is she pooping at the same time as she lays her eggs, or does she sleep in the nest and poop during the night? If it is the second case, then you simply need to block off the nest boxes at sleeping time (with a bit of wood, a beach ball or anything that stops her getting into the nest), and force her to get up onto the perches with everyone else. You need to do it every night for a couple of weeks, but she will soon get the message!
If she is pooping regularly when she lays her eggs then that is something entirely different - I don't have any experience about anything like that, but maybe others on BYC will.
Let us know how you get on with your girl.