Do you think that it is possible?
What you are talking about is called a "flow through" agreement. It is signed by the parent company (legacy) and regional codeshare company. The only one outstanding to my knowledge is an agreement with American and American Eagle, and expires in 2012 I believe. Remember this is a double edged sword. When mainline furloughes 1000 people, they also "flow back" (if they desire) into the regional, displacing those junior regional pilots. I believe this is part of the reason Eagle has such a long upgrade, because all those mainline pilots were given captain slots in the jets (essentially stapled to the top of the senority list).
If the mainline management could have it there way, their company would simply be a "shell" and act like a broker and sub contract out ALL of the flying. IMO we are headed this direction. Less mainline a/c, more whipsawing "regional" codeshares that operate aircraft upwards of 90 seats and more soon enough. Wait till mesa gets its hands on some Airbi!
Most industries seem to follow the practice of promoting from within. In the case of the airline industry, would this be those who are on reserve?
You don't get "promoted" as a pilot, you just get more senior. The most junior pilots at any airline are reserve pilots. Once there are enough people below them to take up all the reserve lines, then you are able to hold an actual line.