Uh, no, WT. There is no negotiating to be done. Rates of pay don't change based on the airplane being worked...
Perhaps this is a change from how DCI is handled, but flights operated on someone else's certificate (including Envoy) aren't worked by AA unless that airline happens to contract for handling by AA. In some places, they will self handle, and there's really nothing stopping them from contracting with DGS or any other low bidder.
There were spokes where mainline and regional were handled separately below the wing. RDU, CMH and STL come to mind, and there may be others. Mainline is rarely the most cost effective provider, since cross-utilization isn't allowed for above and below wing tasks.