RMFRP Arms Law says:
If you are using a “slaying” weapon against a target that is not large or super-large, you should resolve the attack normally. However, if the normal attack delivers a critical, the weapon will deliver an additional critical. Cross-index the roll used to resolve the normal critical on the Slaying column of the Large Creature Critical Strike Table.
Not sure offhand if that also appears in the RMSS printing of Arms Law.
RMU just translates slaying as a bonus on the critical roll, so that would be an easy option too.