Salty is accurate with respect to most current soda fountains, as almost every soda manufactuer is backing the new post mix systems in which you need a water source and a co2 source to mix and dispense.
However, pre-mix soda is still available for more portable events (I see it at conventions regularly and purchased it myself from Coke for an event). Pre-mix is finished soda in a corney keg that you use CO2 to push the soda. No water is needed for a premix system, which makes is veyr useful fo revents where tapping a water source woudl be prohibitive (like a convention at a hotel or serving fountain soda from a mobile serives station at a ball park, for example).
Thus your idea isn't crazy. In fact you've just 'rediscovered' one of the the original purposes of the corney kegs we use.
You could easily purchase soda syrup, add water and carbonate. In fact one beer company that I know of (Sprecher's) sells several flavors of soda syrup by the gallon specifically for home made rtoot beer, cream soda and other flavors using a corney system. Theoretically you could buy post mix bag boxes for all major brand sodas at Sam's or other large outlet and mix it yourself once you find out the ratio of water to syrup.
There's nothing to worry about with bleeding off the pressure. Either pull the corney keg lid blow off valve to vent the excess pressure or, if your lid doesn't have any easy way to do that (I've seen some plastic corney lids that are hard to manually vent) jsut use a phillips scredriver to push in the poppit on the gas valve to vent the pressure.
