I think the armies always made sure they had a clear line of supply/communications. They accomplished this by capturing any castles/outposts en route to their objective.

Automatic replenishment sounds silly (but somewhat appropiate for NTW).

In my eyes Sengoku Jidai can be divided into two periods. The total war stage where Daimyos were busy carving out their own little domain (the early turns in S:TW) and Japan was falling apart and the stage where Daimyos would seek to reunite Japan (when you had your economy running, secured your home provinces and are ready to eliminate other clans).

In the former period few Daimyos could afford standing armies and as such field armies only existed for a short period of time. Replenishment thus only makes sense in the later period when armies where on campaign for an extended period.