From a modding pov, adding those subsystems is easy.
But given the way these systems work in BP fluff (as in, they're integrated into the armor at a very basic level), there is no singular subsystem to take out, no single spot you can hit for massive damage.
As I understood it the delivery system for the liquid is integrated into the armor, but the liquid must still be stored somewhere. If it's stored inside the armor itself, it would either take away a lot of protection (were you have a storehouse/tank you can't have a protective armor layer) or it would seriously limit the available storage space for the sealant. Unless the sealant doubles as the liquid to lessen kinetic impacts, but then repairing the hull would reduce the protection, which would be... not very smart.
depends. If the foam is pressurised and sandwiched between the armour/structure layers what you would get is a passive system for sealing everything from small cracks to probably a mid sized breach while probably providing some shock dispersion in areas that are not or low damaged. if you then fit pressurised tanks to the system it would be able to fix multiple breaches.
Also the armour would be designed with this in mind so either the armour layer would be thicker to give space for the sealant or the ship designers will just accept the reduced penetration protection for the increased capacity of the ship to take damage and remain effective.
