I want to apply CGM this year for fall/winter weed control and overseed with rye grass. Can both of these projects be done in the same year? if so what is the time interval?

Yes, you can apply corn gluten meal several times a year (fall & spring). It is a pre-emergant (& fertilizer); do not spread it before planting seeds. Plant the seeds and when they are growing and well established, it will be OK to use corn gluten to prevent weeds.

On another list I watch one of the gurus there has just reported the findings of his ongoing experiments with corn meal. He's found that using corn meal (not CGM) at the same time as seeding seems to have a preemergent effect, too. So to continue with the thought, I would not use any corn based product when seeding but instead wait after the grass is up, then use your corn based product. You might use alfalfa when seeding.