Dying on Ysgard

In the Planescape boxed set, only petitioners had this ability, and they specifically had to die in battle. I might be more liberal-minded about this in the case of petitioners of less martial powers. Those who serve forge-gods might be resurrected if they die while at their own tasks, for example, rather than fighting. Petitioners of Bast might be resurrected if they die doing kitty things, like laying in the sun or playing with balls of string.

The 3rd edition Manual of the Planes expanded this to include everyone on the plane, as a function of its "minor positive-dominant trait." I'd assume that "dying on the fields of battle" means dying in combat, though. Again, I'd be more liberal-minded in some instances, since there are forms of glory other than fighting. Creating a great work of art, composing a powerful and inspirational speech, running for elected office, defending a case before a court of law - I can see these interpreted as heroic struggles as well.

Being assassinated in your sleep isn't glorious, unless an oneiromancer killed you after a valiant fight in your dreams. I think you'd stay dead. You need to be able to fight back for the plane to find you worthy.

The trait is good throughout the plane, though. Anywhere you fight is a battlefield.

Though not for petitioners, you might expand "battle" to include "childbirth" as the Aztecs did (they believed that childbirth was a battle for the mother). The Aztecs held a similar belief that only those who died in battle would enter paradise, though this was also extended to include those who willingly became blood sacrifices for the sun god (the sacrifices were meant to replenish the god's blood, as he lost a lot each day ensuring that the sun would rise, and thus the world would not be destroyed, the following day)
Most cultures, in fact, hold that those who willingly become human sacrifices will either enter paradise or become demigods. (obviously there are few instances where human sacrifice would be considered a non-evil act, and the rule could be encorporated as simply entailing self-sacrifice, such as say-- a wizard in 2E casting "Transfer Lifeforce" to save a friend at the brink of death, but dies in the process, and knew this either would or was likely to happen when he chose to do it. Another example would be a paladin lunging/leaping/throwing himself at a comrade to shove him out of the way of an incoming stampede of raging bulls, only to end up getting trampled to death himself. Yet another example would be a woman saving children from a wildfire that is now licking their feet by hoisting them up to a safe ledge at about the woman's chest-level. As she hoists them up to the ledge, she burns to death and the children suffer mild to moderate burns *this is based on an actual story that occured in China a few years back.*) All of the instances in parenthesies involve a decision to save someone (knowingly) at the cost of one's own life, yet none of them involve battle. Again, in most pagan/polytheistic faiths, such self-sacrifice generally ensures either entering paradise or being immortalized as the first such-and-such plant, a constellation, a waterfall, mountain, etc by the gods.