You're getting yourself into a downward spiral. You would be much better off not overseeding and working on getting the centipede most dense. The problem comes up next spring when the centipede delays awakening because it is in the shade of the rye grass. If the rye happens to extend into late April or May, the centipede will really suffer. If you can spray something that kills rye and not the centipede in late March, then the centipede has a chance. One of the atrazine herbicides would probably do that for you.