That is one of the most imaginative questions we've had around here for a long time, strictlydating. Thank you. How about this?
1) store-brand ginger ale (I think Seven Up might be too strong) or perhaps store-brand lemon-lime soda; or pinapple soda, as a matter of fact...
2) paradoxically, we probably need just a touch of salt -- this is a usual ingredient oddly enough in cakes and such, as you know. I suppose the sweetness needs to be modified ever so slightly
3) peppermint extract, I should think; or cinnamon (not both, this is serious business, this mouthwash, after all -- we mustn't make this an excuse to simply make something fattening, sweet, and delicious; we're talking oral hygiene here).
4) we need some agent that actually CLEANS the teeth -- perhaps the scrubbing power of baking soda.
A) we would probably need to heat up a bit -- perhaps this will burn away some of the carbonation? (chemistry is not my subject) -- to modify the sweetness a bit more.
B) perhaps, just before we pull the pot off the stove, we should add the peppermint or cinnamon extract, and salt.
C) since we really want that cleaning power of the baking soda, perhaps we should wait until the mixture cools before adding some baking soda -- not too much! Just enough to get that cleaning power! But we don't want the taste of baking soda to be overwhelming.
D) Mix well and refridgerate until ready to use, I think.