At the very worst just charge to 95% (if your charger can do this.) The last voltage ramp up adds hardly any mah to the cells anyway, so if you stop @ ~3.5v/cell you are not really losing anything.
6s lipo is 25.2 fresh off the charger (4.2v/cell). 7s A123 is also 25.2 fresh off the charger...as long as you use a typical CC/CV charge profile and not some "zip cord" charging method. So, it'll work...