After reading this thread I tested this with an architect and a chirugeon ancillary in RTW vanilla version 1.5. I added a new effect for each, "Effect Combat_V_architect" and "Effect Combat_V_chirugeon" and then made trait triggers.

One would give the character 10 points in the IanR trait going all the way to "Narcissist" when Attribute Combat_V_architect = 1 while the other would give 10 points in the Feck trait when Attribute Combat_V_chirugeon = 1 making the character "Irredeemably Foul-mouthed". I picked these because they are really rare. After I made the architect IanR trigger it seemed to partially work although some characters that didn't have an architect in their retinue also got "Narcissist". However when I made the chirugeon Feck trigger I ended up with all of the characters who had either architects or chirugeons, and not necessarily both, being "blessed" with both IanR and Feck so I concluded it didn't really work properly.

Has anyone been able to use this in making ancillaries generate traits so far? I'm clearly not the only one who would love to have a complex mentoring system that would allow me to control and develop my generals up to a point instead of just praying they don't gradually grow into perverted retards...