Hi Dirk,
Yes, it is the best option, while posting from external system, it will call the BAPI, this BAPI calling needs to be link with your Z table, so that what are the days open in a period and fiscal year, system will allow you to post else throw an error message, this mostly you need to work with ABAPER and needs thorough testing.
Else you can try with user exit also, in that you can write this required Z coding.
Regards,
Ravi