In my opinion, I think in the long term it would be easier to make a clean break.  Start with a recent branch, like Jethro, get a minimal image to build for your hardware and then add your own layer with your proprietary recipes.  I think that will be a lot easier than trying to get oe-core working with 5+ year old versions of gcc and eglibc.  Yes, that would mean additional testing, but it may be less testing/integration than you are currently doing just trying to keep your current image maintained.