Interesting approach.
I had some new chickens and kept them seperate for 4 weeks. In the fifth week, I was going to integrate them and one turned out to have Mareks. So now I am keeping the two remaining new ones apart for the total of 12-weeks (on top of the 5-weeks) Mareks has an incubation period of 4-12 weeks.
Each time I think how it would be easier to keep them all together, I think I could risk Mareks (more then they are already exposed since it is airborne) to my existing chickens.
If you have a trusted breeder, and everyone is vaccinated for Mareks, then you could probably safe to combine before the magic 4-weeks. Had I combined at 4-weeks, all my chickens would have been eating and drinking and sleeping with the one that had Mareks. If you put your cull pullet in with them, How long would you wait for signs of illness?
Good luck with the process.