I honestly think the worst problem with putting flavoring in a Camelback is just that the water in the camelback will ALWAYS taste a little bit like what you first put into it.
Even if you add new flavors, the first flavor doesn't go away. So if you used ice tea at first, then changed it to fruit punch, it would taste like ice tea & fruit punch. By the end of the week it might taste like ice tea, fruit punch, lemonade, orangeade, and tropical mix all mixed in together. The last flavor would be stronger than the others, but the first flavor(s) would still linger in the aftertaste.
I don't think the sugar and dye and crap in the flavoring will hurt the Camelback at all. However, depending on what you use to flavor the water though, it might make you thirstier. Kool-Aid would be a bad water flavoring choice, as compared to something with electrolytes in it like Gatorade.
Given the choice though, my personal preference would be to have a container of just plain water, put my flavoring of my choice into a cup or bottle which I use for drinking, then add the water to it. A variety of flavors to choose from with every drink of water, no lingering after taste.
Something like a camping cup of your own is useful at the Central cafe as well. Those that bring their own cups for coffee are given preferential treatment. You can even eat soup out of it if it's big enough.
That way I would have a choice of lemon flavor, orange flavor, etc. rather than just having a Camelback full of Orange flavored gatorade to drink all day long. (Which might get as boring as just plain water.)