modbus: Is there a way to have access to unit_id when using MODBUS_RTU mode?

[lucas-ros]

Using the modbus rtu server implementation, is there a way to access unit_id? I have 2 modbus interfaces and I need them to have separated access control. In order to do it, I need to have access to the unit_id during the write registers callbacks, so I can verify if that interface has the right access level to write to the register.