There seem to be three aspects of learning absolute pitch: chroma isolation, categorical perception, and structural decoding. If all three of these can be taught, it may result in true, musical absolute pitch.
1. Chroma isolation is required to make an absolute judgment of musical frequency.
This ability seems to be what's lost after the supposed critical period. When a child is naive about what he's meant to be listening for and how he's expected to interpret music, using absolute chroma to interpret musical sound could be no more difficult or peculiar than listening for traditional tonal relationships. An adult has learned to integrate and ignore chroma to make relative pitch judgments.
Absolute Pitch Blaster, by a perceptual differentiation strategy, teaches chroma isolation. One down, two to go.
2. Categorical perception is necessary for chroma to be recognized by its functional identity, not by its relative or subjective characteristics.
Chroma isolation does not lead directly to categorical perception. Naming chroma accurately is not an absolute-pitch judgment if the judgment is made based on perceived sensory qualities such as lowness or highness, descriptive adjectives, emotional effect, or similarities to other chroma. As with visual color or language phonemes, objective identity is perceived and labeled first; qualitative and subjective judgments are generally made upon reflection.
The most exact model for categorical chroma seems to be color learning. It might be possible that colors are learned by mere repetition of exposure to name-color pairs, but that would provide no explanation of why a child learns colors categorically, rather than "higher" or "lower" on a spectrum. Categorical perception must be induced by the tasks for which colors are used-- and colors are typically used for color coding. Red traffic light means stop, green means go. Differently-colored apples promise different tastes. Identical objects convey different meanings because of their color; and, because we recognize objects by learning their distinctive features, learning to recognize these objects requires learning to identify color.
To solve the problem, I will want to create tasks that require a learner to regard different chromas as functionally distinct. Tasks must be invented for which A-sharp means one thing while D means another. The task might be something as simple as labeling objects with different icons like Chordhopper already does, but a kitchen-sink approach may be necessary to induce categorical perception-- perhaps using microtones, octaves, timbres, tones, chords, key signatures, intensities, and more, so that chroma is a definitive feature of each object but offers its own peculiar representation of the category.
3. Structural decoding is required to give musical value to absolute perception.
It is no more effective (or possible) to listen for individual notes in music than to listen for individual letters in language. However, individual letters are learned from exposure to them in language. Once you're aware that the letter P exists, you're going to learn it more readily by hearing Peter Piper picked a peck of pickled peppers than you are by comparing pig and big. If you compare big and pig, perhaps you would learn that P is not B, and you might even become able to recognize a P versus a B, but hearing the Peter Piper sentence you'd only be able to tell that there are some P's in there somewhere. You wouldn't know where they were, or what function they served, or how to write out the sentence.
This will require nothing less than constant decoding and encoding of musical structures at all levels, from interval to sonata. I'll have to tackle this last because I don't yet know what the structures are myself. I also don't know whether it's possible for a computer program to easily generate an infinite number of legal melodies and phrases even with substantial data entry, but if there's a way to do it off-line or with MIDI instrument coaching I'll find it.