Well, I'll post my version of this sort of spellcasting, through a core class I made called the Magus (plural Magi) for a dual setting I call Convergence (high-tech world in a scientific setting with no remaining magic is connected by gates and portals to a high-magic world suddenly, and magic is rekindled slightly in the high-tech world). Haven't finished enough yet on the setting and the handful of new feats, classes, and gear (for characters of the high-tech world) to make use of the setting yet, though.
My setting uses a sort of Wound/Vitality, but with Health replacing Wounds, and since I don't have UA or the SWd20 books, it isn't directly based on that system. Still, it has similarities I'm sure, so this should be convertable to the Wounds/Vitality system in d20. For reference, Magi get d10 hit dice normally (so 1d10 added to max Vitality each level), and are capable of being either decent in combat (through little or no spellcasting), or poor in combat (with enough spellcasting to be similar to a sorcerer, or maybe a psion in equivalence). Their spells known are similar to a sorcerer in number, but a bit slower and more linear in acquisition (like a wizard, starting with some 1st and 0-level spells known, gaining new spell levels at the wizard's rate). Magi get slightly better familiars than normal, and a few bonus feats over time, with an average BAB and little else in the way of benefits or abilities.
Magus spellcasting details in spoiler block.
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Spells: A magus casts arcane spells from the Magus Spell List (see below). He casts these spells without needing to memorize them beforehand or keep a spellbook. The number of spells a magus knows are determined by his class level, as shown in the Magus Spells Known table. Magi do not have spell slots like other spellcasters do, and instead of using up spell slots to cast spells, magi expend small amounts of life-force.
Thus, whenever a magus begins to cast a spell, his current Vitality is reduced by a number of points equal to 1 + the spell’s level (or it’s effective level, if modified by metamagic; though magus spells still have the same limits to their effective levels as those of any other spellcaster). If the spell’s level or effective level is 3rd or higher, then for every 4 levels or effective levels of the spell (considering metamagic adjustments to required spell slot, which function for a magus as simply an equivalent increase in Vitality or Health cost), the magus may instead reduce their current Health by 1 point, in place of the 4 Vitality they would have otherwise spent for the spell.
Note that these reductions in Health and Vitality do not count as actual damage, and thus the magus does not begin to bleed to death after spending Health to cast a spell; however, they still suffer all the normal penalties and restrictions inherent to having their current Health or Vitality reach certain points, they just don’t begin bleeding to death after spending Health to cast a spell, because they pay in life-force rather than actually being injured. Note that the magus can also be healed after casting spells, such as a friendly cleric casting Cure Light Wounds upon the magus, and thus be able to cast more spells afterward, in such events.
Also, a magus cannot cast any further spells if they would reduce either his Health or Vitality below 1 point; the magus cannot willingly, or even accidentally, cast so many spells to where they would kill him or knock him unconscious. However, due to the special way in which they cast their spells, the number of spells a magus can cast each day is limited only by their Health and Vitality and the amount of healing they receive that day.
Magi receive bonus spells for high Intelligence, and to cast a spell a magus must have an Intelligence score at least equal to 10 + the level of the spell. Note however that due to the special way in which magi cast spells, their bonus spells for high Intelligence instead affect the number of spells the magus learns of any given spell level at the time they gain access to it, rather than allowing them to cast more spells per day; this is a special exception for the magus, since magi don’t have spell slots and cast their spells differently. The Difficulty Class for a saving throw against a magus’ spell is 10 + the spell’s level + the magus’ Intelligence modifier.
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I can see that this might make spell caster weaker over-all, and that is acceptable given the other campaign parameters.
