Half my family is Indian (Sikh) and I was exposed to this much before Buddhadharma. When I first encountered Dzogchen a couple years ago, I think via a discussion on E-Sangha, before I ever even took refuge, I remember thinking, this Dzogchen stuff is vaguely reminiscent of Guru Nanak's teachings. Later on, I read he was considered an emanation of Guru Rinpoche as well, and practicing Nyingma, I thought to myself, wouldn't be a karmic stretch for me here, would it?
I don't know enough about Dzogchen to comment on the relationship, but at least from personal experience, they smell similar in essence, while the wrapping might be presented very differently. But again, I am very ignorant and could be drawing parallel through my lack of knowledge. It's just curious you created a thread on this, as it's something I've wondered about myself..