I think a mod should only broadcast own casts to prevent unnecessary spam.
As far as the library goes, someone should look into IsCasting by AnduinLothar (here WoWI).
Add some way to filter the spells and a communications portion and I think that gets it started nicely.
I think it should be up to the individual mods as far as the format for storing the data is concerned. While I'm sure we could come to some agreement on a format, is it really necessary when we could call functions with the paramenters passed to the function.
Just my 0.02