ZDoom

Virtual function in which can be defined how much time some actor receive damage, like burning, radiation or poisoning, with pointer to actor which inflict such damage.
Like "DOT(Damage Over Time)Mobj(actor source, int damage, int duration, string damage type, other important variables)", i.e. wrapper around damage mobj which calls it some time, if that is correct name for this.
Because if I want to make a projectile which must cause internal trauma/bleeding/acid/alkali/etc. damage I must use hacks with inventory items and/or projectiles which perform such attack to correctly store pointer to actor which perform this attack.

You do not need hacks. What you need is a thinker that periodically inflicts the damage. Internally I couldn't do it any other way.