Can neutralization be reversed? So far, maybe it's just that my course isn't advanced enough, but I have never seen anything called 'neutralization constant'.

Any such reaction involving an acid and a base will be complete no matter the strength of either reactant. 

Buffer works the same way. If the reaction is reversible then the buffet is not effective.
The reverse reaction of neutralization is called hydrolysis ( X salt + H2O = acid + base )