No, I have never heard of a non-buddhist obtaining rainbow body, except Bon people. But Bon has exact same buddhist view, intention/aspiration, dedication of merit etc. And it is these things that make the difference
Do I think it is possible for a non-buddhist/bon person to obtain rainbow body?
Also no. If you understand how the rainbow body process works, you know you need dedicate merit, have the proper intention to benefit all sentient beings etc.
I think its important to understand that in Indian tantra, whether buddhist or hindu, it is said that you can use tantra (yes even the AYP stuff taught here) for any purpose including evil, material wealth etc!!
The intention is part of what lets you obtain rainbow body or whatever lower spiritual goal you have. The other part is view. I tried to explain this before.