From my previous experience I can say that doing
oMatrix.Addrow();
on a system document form
does not cover all the features of the UI form, for example the Addrow() method just add an empty row and does not initialize the % discount column wich is prefilled in the new row by SAP without exposing the api for doing such as initialization.
Anyway I found totally unreasonable to have to apply a similar solution for a so trivial need.
regards
D.