Well first we need a system that actually supports selective regeneration over general regeneration which our bodies aren't currently designed for.
Stem cells are one of the principal components in regeneration being able to transform themselves into a variety of destroyed cells. One organelle in particular that is exceptional in producing and distributing stem cells is the bone marrow. Bone marrow is also what produces platelets which are responsible for clotting and forming the scaffolding needed to repair tissue.
Next we need to detect where damage is that needs to be repaired. Fortunately the human nervous system is fairly good at this as that is one of its primary responsibilities.
So to make this possible the nervous system needs a connection to a construct near bone marrow locations that can increase the metabolilc process of that bone marrow site. That would increase platelet and stem cell production. The triggered bone marrow site would be located upstream (bloodstream) of the injured area.
This could be designed to be autonomous or cognitively activated.
Is this feasible? sort of , is this controllable? not entirely as it is just increasing healing in general areas.
If you need more control, speed , and precision I suggest just resorting to nano machines in the blood.
How fast can it heal? I would guess the max speed any penetrating wound could be healed in would be roughly 4-8hrs. That estimate is fairly handwavium as you would need to tweak the metabolic speed of the cells themselves. Without that you would be looking at days since the full length of mitosis occurs in roughly 16-20hrs.