I've never seen any of my roos or my drakes eat the oyster shell. Thats not to say they may have tried it and I didn't see them, but I doubt they would make it part of their diet.
I like Celtics idea too, I feed Purna Flock Raiser and just started adding other grains with help from Celtic about what grains are the best, and it is working out great, so far I feed the FR and add a cup of mixed grains [Whole oats, barley, and wheat and a 7 way scratch ] top dress the FR with...