Hello,
In principle there should be no reason for not splicing a piece in there even if it will be loaded. That said, the splice alone is not the sole determining factor to consider.
In this situation certain splicing joints can be ruled out, for example, half lapped and splined.
Of course extra support is always possible whether it is from 2x4 or something tenoned in there between the girt and sill.
Personally I have not thoughts on books of the type you mention.

Greetings,

Don Wagstaff