Heh, I intended to use Bloodied + Second Wind in my Eberron game as well. This is my version so far...
“Bloodied”
When a PC, NPC or monster reaches ½ his normal hit point total, he becomes bloodied. When a creature becomes bloodied, it is immediately noticeable to anyone around them. It can manifest as some non-serious wound, scratches, burns or bruises, or simply being winded. Becoming bloodied has no immediate game effect, but it does open up additional actions and abilities to use on yourself or your foes.
Even creatures that do not have blood or even physical bodies can become “bloodied”, since the term is used to represent amount of hp, not actual blood or wounds.
The bloodied condition persists for as long as the creature is below ½ his total hp. Any healing that brings him above ½ his total ends the bloodied condition.
Only regular hp damage invokes the bloodied condition; ability damage, level drain, and other non-hit point attacks do not trigger the bloodied condition.
Beyond “bloodied”, there is no way to determine a creatures remaining hp in combat, this goes for PCs as well as NPCs and monsters. For abilities that rely on a fixed hp amount (such as a paladin’s lay on hands) actual numbers can be exchanged, and PCs can determine exact hp amounts lost outside of combat with a successful heal check (DC 15).
Second Wind
When a character becomes bloodied, he can draw upon his inner reserves to press on. Once per day, he can heal himself ¼ of his hp total as a swift action. He can use this ability if he is already in negative hp (-1 through -9) but not if he is at -10 or less hp.
Only characters with PC classes can gain a second wind. NPC classes and creatures with monster HD do not. The exception: characters who take the feat extra second wind gain a use of this ability once per day.
Extra Second Wind [General]
You are resilient and always come back for more.
Prerequisites: Endurance
Benefit: You may use your second wind ability one additional time per day.
Special: This feat make be taken more than once. Each time you take it, you gain an additional use of your second wind ability. Creatures which cannot use second wind can take this feat to gain one use of the ability. This feat is a fighter bonus feat.
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I have a variety of other feats that tie to this condition as well. I removed the "or con" element since that seems more for Saga's x3 hp rule than D&D's current hp rule (and it would make second wind effectively a full refill for most PCs levels 1-6 or more)