I will just put Heed directly in the bladder for my Camelbak. I did this last summer for rides 3 hours or longer and never had a problem with things growing in it. I might get a separate bladder just for this and make it live in the freezer when I am not on the bike - but the new Camelbak bladders are much easier to clean and the material is supposed to have anti-bacterial properties.

I also decided against a seat-bag, I've got plenty of storage in my pack, more than enough that provides room for tire-changing things + lots of other stuff that I might decide I need.