I was given a broody with eight eggs, she is down to 2 eggs, has pushed the others out (knowing they are not right I suppose)  we gave her a little pen seperate from the new flock she will integrate to - it has  a covered nest box area and a little  4'x4' play area (aprox. may be bigger) anyway plenty room for mama and chicks.....I will keep her in with the chicks feeding all chick unmedicated starter....the flock will see her as one of their own because she has been sitting on the eggs in plain sight I really don't know when day 21 is but I have had her since the 1st of Jan.....was so happy she continued to set after the move......she gets off the nest to feed - look around - stretch her legs (ha) then goes back to the 2 eggs.....looking forward to the experience of mama with chicks.....the only other time this occured - a hawk killed mama (she must have sacrificed herself for chicks, raised the chicks in a brooder after.....    sorry so long winded   .... 
Bloomchicks what a great idea for your door....such a neat set up....would it help to put chicken wire just high enough that the chicks can't go out - until they are ready to do so, the hens will hop over the the little "gate" in the door way...there looks to be so much room....great coop lucky chickens