Calculating residuals with specific lags in ARMA/ARX etc

I am reworking some models where I know the specific AR and MA parameters. For example one model has only a 7th lag AR and a 7th lag MA (so no values at 1, 2, 3, 4, 5 and 6). If I use the standard routine (ag ARX(data, [7 0 7])), it will calculate values for all 7 AR and MA parameters. This generates a noise sequence, and is different from the sequence that would be generated without the non required values (i.e those for terms 1, 2, 3, 4, 5, 6), even if the values of the non required terms are small.

I thought I read that version 2011a would provide functionality for this, but cannot find any reference for it. This question has been asked by others for previous Matlab versions.

Thanks for anyone's help - Dave