sorry can't really help,ours is like that most of winter, but our hens free range, so they just go to bed with muddy feet and paddle for the frist hour or so in a morning.  Drainage sounds the best option, but it's a bit of a bother, you could just put a few big branches or logs in the pen, so that at least the hens havesomewhere dry to perch when they feel like it - we have several big old branches and they love to sit on them.