Muramasa was never a toju to begin with. And since Koga never learned his bankai then Muramasa wouldn't have been able to acivate it so the dome couldn't have been a bankai. but sense muramasa out lived kouga didnt that make him a toju at the very end i mean it is possible, after all toju are zanpacutou who have lost their wielder and in doing so they are in constant bankai.