Im not sure if they are removable or not but maybe you could give them a water blast? (I think they are also called guerneys?). That would get off the majority of the dried on poo and get into the cracks too. Im sure domestos would be fine to use, but it smells quite strongly. Would that put the girls off going back in there to lay I wonder?
For your own health its a good idea to keep the laying area as fresh and clean as possible. Eggs are quite porous and you don't want them absorbing bacteria from poo and then you eating them. Even having to wipe poo off eggs has the potential of having the egg absorb liqued which you don't want either! Ewwwww! If you plan in incubating the eggs you'll have more success with clean eggs too.
