I think the person that theorised that Hamura is the reaper god might actually be right the may use hamura or his jutsu to seal Kaguya.
If that were the case than normal people like Minato, and Hiruzen wouldn't be able to use RDS without the yin, and yang seal. Though Hamura may be RDS, I don't think that the seal they will use. They'll use the same seal that Hamura, and Hagoromo used on their mother. Which couldn't be the RDS, because Hamura would've still been alive.