In esoteric cosmology, a plane, other than the physical plane is conceived as a subtle state of consciousness that transcends the known physical universe.
The concept may be found in religious, and esoteric teachings - e.g. Vedanta (Advaita Vedanta), Ayyavazhi, shamanism, Hermeticism, Neoplatonism, Gnosticism, Kashmir Shaivism, Sant Mat/Surat Shabd Yoga, Sufism, Druze, Kabbalah, Theosophy, Anthroposophy, Rosicrucianism (Esoteric Christian), Eckankar, Ascended Master Teachings, etc. - which propound the idea of a whole series of subtle planes or worlds or dimensions which, from a center, interpenetrate themselves and the physical planet in which we live, the solar systems, and all the physical structures of the universe. This interpenetration of planes culminates in the universe itself as a physical structured, dynamic and evolutive expression emanated through a series of steadily denser stages, becoming progressively more material and embodied.
The emanation is conceived, according to esoteric teachings, to have been originated, at the dawn of the universe's manifestation, in The Supreme Being Who sent out - from the unmanifested Absolute beyond comprehension - the dynamic force of creative energy, as sound-vibration ("the Word"), into the abyss of space. On the other hand, it states that this dynamic force is being sent forth, through the ages, framing all things that constitute and inhabit the universe.

Une autre proposition..

A mon avis, il faut garder cette idée de l'esprit dans un avion, en altitude, en élévation, là-haut.. Un esprit qui voit, qui contemple les choses, sans pouvoir les analyser. Comme l'explique le texte ci-dessous, il faut différencier 'the mental plane' du 'mental train'...

"Or hop on a mental plane
I don't stay far, but this a place you can't get to in your car"
"Elzhi’s second mind traveling option is another play on words. You can travel by train or plane. Here Elzhi refers to a “mental Plane” which is the universe made up purely of thought

An airplane is very impersonal and very fast. You don’t see where you are closely only from up high. It doesn’t let you interact or analyze things as opposed to the train"
http://rapgenius.com/Elzhi-memory-lane-lyrics#note-245594

Un petit exemple de l'emploi de "esprit en lévitation"..
"Ce mirage de havre de paix n‘est retombé qu‘avec moi. Une fois de trop ébloui, l‘esprit en lévitation voguant entre les nuages, dans un trou, bien noir. Oui, la béatitude et les rêves font souvent oublier que nos pieds doivent toujours rester, eux, en contact avec le sol."
http://remitaines.com/photo/blanche-fiction/

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Note added at 16 hrs (2012-05-20 09:47:52 GMT)
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Ou peut-être même quelque chose comme "perché dans mes pensées", qui fait ressortir cette capacité de percevoir des choses d'un endroit élevé, mais dans l'incapacité d'intervenir.

I honestly think that 'plane' is simply 'au niveau de...' here, nothing too lofty. He's describing what had been trotting around in his head (in his imagination) to inspire him.

To back my idea, found this:

"une étude a mis en évidence le rôle fondamental de l'évocation comme outil de la pensée. L'évocation est une image mentale, visuelle, auditive ou verbale, ...


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Note added at 17 hrs (2012-05-20 10:49:40 GMT)
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Perhaps, on reflection "dans mon imaginaire...." he's wanting it to sound a tad lyrical.....

Il faut préciser qu'il s'agit d'un des plans de la conscience (ésotérisme, spiritualité, théosophie....) et non de la capacité mentale au sens cognitif.