Tony:
I came across a reference to book entries in an on-line thesis 'Playing at command: Midshipmen and Quarterdeck boys in the Royal Navy 1793-1815'
The author mentions that captains falsely entered names on the ships' musters either to advance family members or as an exercise in patronage. This was, as you say, technically the crime of 'false muster' but 'as long as the captain did not draw pay for the absentee 'able seaman' or 'master's mate' the Admiralty turned a blind eye.'
see
www.etd.lsu.edu/docs/available/etd-0331 ... thesis.pdf