To me, an interesting subject, Sylph.
I've pondered a lot on why people compose music and how that fits in with their creative energies altogether - indeed, what these energies are and where they come from. Some composers would seem to tap into their inner Mysteries and somehow convert what they find into music. Sometimes the process involves filtering certain experiences of their existence. Problem is that intellectual discussion is thwarted by the absence of an experiential vocabulary ~ the data always remain private.
The question of trying to approach via the occult route is that many composers penetrate their depths without any awareness of "occult techniques". Highly probable that occultists are aiming to achieve what artistic creators of the spiritual sort seem to achieve naturally.
Point is, some composers are/were active occultists in some form or another (such as you describe Holst, concerned with Mystical rather than physical planets - or Satie
Soverign Templar of the Noble Order of Umbrella Collectors); many aren't but find some way into their deeper selves in pursuit of music, e.g. Debussy, probably via the booze, opium and sex. .There are some - the form-fillers - who 'create' merely by doing what's already known to work. And there are those who simply were unable to reach the interiors they so desperately sought but still tried: to me, Mahler comes into this category.
It's a subject as big as you care to make it.
PS, Though not strictly an item for this forum, have a look out for an Electroacoustic piece by an ensemble known as Current 93, the work: Live at Bar Maldoror... One of those entirely unique works created by people immersed in the occult.