Quite common to record flight time in the MR and charge student VDO.
As for running over the MR - as I am aware the only time that might be legit is if there is
unforeseen circumstances during a flight that was due to finish prior to the 100 hourly.
Eg. 2 hours left to run, you may do a 2 hour flight planned flight if there is no notammed traffic holding or weather holding. If you are radar vectored for controller amusement or the like and end up at 2.1 then that's fine.
As long as the original flight time (plus foreseeable delays like TEMPOs) was planned to be back before expiry you are covered.
Cannot recall the reg though....
Not uncommon to have CASA approved maintenance systems which have 120 hours between inspections. However this 120 limit is written into the MR and is an upper limit.