Here are a few hints that I've given in previous group hatches, some for those new to incubating, others just asides:
I do increase humidity at the end of my hatch, as per my Incubation Cheat Sheet, the same for Marans as for other eggs. My feeling is that if you have evaporated the proper amount during the first 18 days, you can't drown them with humidity at the end. Humidity merely prevents evaporation, it doesn't make eggs 'take-on' water.
I use an emery board or 400+ grain sandpaper. I use a circular motion and make sure I'm moving the hand with the sander, not the egg- then put the eggs on a paper towel and mist them with a 30% povidone iodine to 70% water blend. I then wait for it to dry, turn, repeat, dry, turn, repeat. You can't be too careful about making sure the whole thing gets lightly misted, but don't saturate it.
This gives me time to turn (if need be), smell, and candle my eggs if I like. I just open it for a few minutes if I'm not intending to handle them. I do this most days until 'confinement.' Some will be aghast and argue this is insane behavior...but I'm just sharing what works for me.
I have used canned tuna to boost protein quickly, and the birds adore it. If you can find turkey or gamebird STARTER, that tends to be around 30% protein and you can mix it with scratch 4 or 5:1 to give them something fun to eat and still have overall higher protein. This also works if you mix wild with gamebird starter- and wild bird mix has a much lower corn content, so you'll be diversifying their feed better.
You can also use black oil sunflower seeds to treat them, as it's about the right protein content. If you have a feed mill near you, ask for 5# of 'peanut pickouts' to take home and treat the flock with- this is just raw unsalted peanuts, which are exceptional for protein and amino acid profile for breeders. Much cheaper from the mill
A small dish of sand, an old branch of fallen wood, and a plug of garden soil with grass (roots and all) are excellent things to add to the brooder for entertainment as well as microbes and appropriate environmental exposure. If you have these and you keep your heat source in one corner so that the chicks can control their own temperature by going nearer or farther, you won't have any issues with mucky bums at all.
This works well for me, but like most on BYC, I don't consider myself an egg-spert... I just like to share my egg-speriences to help anyone else who might be able to learn from them!