Here is a simple variant to burn abstract Hit Points in melee combat I'll be using in my Undercity of Mog campaign.
A PC/NPC/Monster may sacrifice HP to inflict extra HP on a foe. Before damage ir or the to hit roll is rolled the combatant should announce they are burning HP so as to inflict more damage on a foe on a one to one ratio (if the attack hits).
These burnt hp reflect fatigue, stress, minor nicks, bumps, and scratches suffered while belaboring an opponent in combat. The character burning HP loses them if the attack misses or hits but the target only suffers the additional damage on a hit roll.
No Ac adjustments no change in the to hit chance the player knows exactly what they risk and are putting on the table.
A limit of 3hp per level is suggested for Fighters and 2hp per level for all other classes/monsters. Fighters, Assassins, and Monsters can burn hp into the negatives if they start with a positive hp total... this is of course very dangerous but also provides an opportunity for a defiant dying blow of sorts.