I don't know of any research, Justin, but thanks for asking the question. It's stimulated me to find out more about the man, his context and his work.
You might already have done this but if not it might be worth asking Alan Senior of the Inverness branch of the Theosophical Society who has been researching Charles Rennie Mackintosh's esoteric interests and has written about them. This is I think his up to date email address,
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Dr Vivianne Crowley
Pastoral Counseling Faculty
Cherry Hill Seminary