It was an "interpretation" to B31.3 that introduced this idea, bringing B31.3 in-line with B31.1.

Can you apply this to B31.4 - if you agree with the interpretation and it is acceptable to the client/inspector.

Can you use a value of 0.00001 - that will effectively remove the SIF from the equation. One can argue that the SIF is for cyclic loads, and the Sustained case isn't cyclic. However, the Codes are introducing "stress index values" in place of SIFs. I would not use the 0.00001.