Coachbyrd,
We've overseeded this season with a rye that I really like. The salesman who calls on us said he had some rye designed for overseeding athletic fields that was relatively, slow-growing and clean. Our field looks great right now.
It's Barenburg Advantage SOS (super over seeding) 400 and the label says its 98% pure seed.
I don't know it you can find it in your area (we got ours through BWI)but its worth looking for. I'm not sure if it's perennial or annual.
Last season we got our rye from the local farmer's Co-Op (perennial for the IF grass, annual for the OF) and it had so much poa anna in it our infield grass looked like a bumper pool table!
Hope this helps.