FINALLY, we have a breakthrough!
See this new thread: LINK
Lukas Jezny developed an application for a Photon, in which he implements a HAP (Homekit Access Point) on the device itself. So, you don’t need a Homebridge or an OpenHAB server to control a Photon with Siri… 
It works beautifully: You compile a sketch and flash it to any Particle device and then it can be directly paired with Homekit on any iPad, iPhone… Currently, you can only control the Particle’s RGB LED ON/OFF state.
I was hoping that Lukas would also show us how we could access Particle.function s and Particle.variable s from Homekit. But, fair enough, he apologised because he can’t make the time for it. Pity…
He shared also the Apple document explaining how to expand the functionality, but it goes over my head!
If some of you, professional programmers, would be interested to try and expand his work, I believe many Particle users would be very happy! I would… 