Good question, ProphetSword. Only slowdive/Jer can tell you for the planned default IB setting.
That being said, it should be possible right now already to simulate an unconscious at 0, bleed for 1 point per round when hp are negative and dead at -10 system.
Dropping down at 0 or below is the case already (while treating the pc internally as dead though).
Now, one could use the OnStartCombatRound script hook of an encounter and assign a further point of damage per round to each pc who is at below 0 hp.
The scripts for the healing spells / heal traits / heal skills as well as the restSystem.cs could easily be modified in a way that they only heal characters anymore when their hit points are above -10 and only "revive" them (change char state from dead to alive) when hp reach 0 or more after applying the heal effect.
In character screen the written state of the character could be made dependent on the number of current hitpoints via the script characterSheetData.cs: this way the player would perceive the internally already dead pcs just as unconscious unless their hp are at -10 or below.
Haven't tried this so far, but I would be rather optimistic for it to work out (or so I hope :-)).