I'll reserve judgment on whether or not Dawn's role has been increased permanently until a few episodes after this passed one with MLK's assassination.
The Civil Rights movement and the Women's Lib movement had closely related trajectories, yet from the very first season, we learned all about Peggy's backstory as she rose from secretary to copy writer (still rare at the time). Why is it so difficult to do something similar for a black character like Dawn?
This is true, but Peggy got in through the backdoor as a secretary. That was an in.
We know the people who have the power to hire at Sterling Cooper are not exactly the most enlightened. They backed themselves into a corner at the beginning of the fifth season and the only reason they hired Dawn was for show, so they could say how "progressive" they were.
I wonder what capacity a black character would've been hired in at Sterling Cooper from the beginning of the series?
To me, it seems as if Matt Weiner thought he could get away with having either a black or woman copywriter but not both.