Reading the Blood Sorcerers of Paja, in a roundabout way this mention of using human flesh for magic, got my mind meandering back to an old line of text:
"Connall took out the brains of the dead king and mixed them with lime to make a sling-stone - such "brain balls," as they were called being accounted the most deadly of missiles.  This ball was laid up in the king's treasure-house..."
 - Celtic Myths and Legends, T. W. Rolleston
I've been cludging away at some ritual rules for awhile:  something like spells, but taking more time, consuming more components, etc. but with the advantage of being usable by anyone, and not consuming daily spell reserves.  Anyhoo...

Rituals consume components, and specific components can be tricky to obtain.  Stealing the shadow, cast by moonlight, from the third-left-leg of a man-sized spider, can be an annoyingly specific proposition.  Do such things even exist?  And who knows how tough it is to find some sort of spell or ritual to steal a shadow.

Fortunately, some genius figured out that there's a universal substitute, at least for arcane spells.  It turns out that the brain of a magus, properly seasoned and desiccated, provides a ready substitute for the trickier to obtain materials in a number of distasteful rituals.  The arcane convolutions of it's folds and synapses, built up over time, work as a powerful catalyst in reshaping space/time on a fundamental level.

This insight quickly created a bloodthirsty economy of greed and fear among magicians, many eager to tear apart each another's skull in a bid for quick power.  The eldest sorcerers, traditionally the most powerful, became those with the most to lose, as their own crania took on a monetary value equal to thousands of years of peasant wages.

The Daybreak Kingdom quickly restored order within its own reaches by declaring all cerebella magicae to constitute Crown property, and vigorously enforcing this dictate.  Under Kingdom law a magician's brain is entrusted to his care and keeping during his lifetime.  But upon his death, all claims on its recovery and possession revert to designated government agencies for stockpiling and use in times of war.

All magicians in the Kingdom are technically sworn to uphold these measures, and most do so wholeheartedly.  But some black-market still exists for the brains of off-the-books hedge wizards, obtained through more or less violent means.  And beyond the control of the Daybreak Kingdom's Great Houses and patrols, some unscrupulous sorcerers and bandit-kings still pose a threat to magically inclined individuals traveling in the hinterlands.

The preparation of a magician's brain requires a week of soaking in various caustic solutions and herbal concoctions, but costs the mere pittance of 100gp.  Once prepared the brain can be used any of the following ways:

Spell Slots - By ingesting a number of doses of the brain, a caster may recall ANY previously cast spell and cast it again immediately.  The number of doses derived from the brain is equal to the original owner's caster level squared.  The number of doses used to cast a spell is 2x the spell level.

Spellpoints - Spellpoint systems work similarly to the spell-slot system, though a single dose should be regarded as 1 spell point.

Magic Items & Rituals - For the purposes of calculating costs in rituals and magic item creation, a magician's brain is worth 10 x 10^(magician's caster level)gp.

Hedge Wizards - Magicians who only use magic through rituals still have some small value to their brains.  To determine their effective caster level, take their Esoteric Lore skill level and divide by 10.  In "OGL Fantasy" games Knowledge (Arcana) or Spellcraft ranks (whichever is higher) may be substituted for Esoteric Lore skill level.

Edit 10/13/14:
Self note:  Although illegal brain trade could be a serious issue, the brilliant idea of "Wizard Heads" would provide a great public excuse for turning wizard brains over to the authorities ASAP.