Now I see that Jason Miller works with Tibetan entities, but am looking for sources on how to work with them. Anyone have any ideas?
A good Tibetan Lama would be the best source.
You could approach the entities through Jason Miller’s work and ask them to teach you how to work with them traditionally, if that is what you are looking for. The gods themselves can lead you to proper sources for knowledge. Miller was taught by Tibetan lamas so his stuff is legitimate for approaching the gods.
I do know Tengu is a multi-armed Tibetan demon. Concentrating on mandalas for Tengu or whatever else.
When working with any entity, it is important to get to know them energetically, first, and theoretically, second. Feel your way around them, before so much as googling their name. Compare and contrast, as the former lets you see their true form unabashed by the pomp of the monkey mind, but rather, in energetic expressions, whereas the latter is what adds the pomp of human culture to their skin, fleshing out what started out as an impersonal energetic impression, into an interactive interface. Extend the hand to the spirit world by way of meditation, or offering.
The above holds true regardless of tradition, culture, practice, theory, framework, religion, group, schema, or philosophy.