The point wasn't that GCal solves your problem. The point is that you should first look at purchasing a solution rather than developing one because it's almost always going to be much cheaper for your employer to do so. Often, the difference is on the order of five to ten-thousand dollars (or euros or whatever).

I have no idea what kind of applications are out there, other than Exchange/Outlook which does provide this kind of functionality. I'm sure there's other stuff that will do exactly what you want it to do.