I'll ride with a camelbak that fits under my jacket to stay hydrated, when I am doing resort stuff and not always. It's more of a skin than a backpack though.
I also ride with a camelbak to stay hydrated and wear it under my jacket, but it looks like I have a humpback. Do you have this problem as well or is your camelback that slim? If it is that slim do you have a link or photo of which bak you use?
*When you look good, you ride good.*