The only thing you risk is if your admin staff regularly put in manual attendance or if your system is set to update registration marks automatically from periods.
The first can be dealt with via a polite conversation with the aforementioned staff.
The second can, if its in use, be switched off. Only the school can tell you if they are using that option.
if the bromcom system is set to update registration marks automatically from periods, if there is no period mark entered (because the tutor is using Lesson Monitor) does it write back a no mark for Registation and copy it back to SIMS?