Well I think we can try and use the discrete states available in s-function ( I am using this since I don't know stateflow yet) and check which state the system is in!
When in the required state (i.e needing 8 bytes) it takes data from buffer, Now in buffer we can choose to insert say 'N' if no data is available. So, after the buffer is full we can check if anything other than 'N' there in the array, if yes we increment the state.