While interesting idea, I think it would totally screw AI.
There is no commands saying to move unit X send family member and take the unit with him and this means that AI will be moving only with family members and units that accidentally have been trained in city from where he started moving.
Is this a confirmed phenomena or merely an educated assumption, If the latter then it surely bears at least some experimentation or all we know the AI may react quite unexpectedly faced with such a restiction. But before any descion is reached a certain degree of empirical knowledge is a must.

lets not forget that large empires tend to have around 1 general per 2 provinces anyway.
Not if generals are recruitable.

So the solution which i can't believe nobody has yet mentioned is as follows:

Captain led armies can move only in provinces that you own, but can never enter enemy/neutral territory without a family member
Motion seconded