Product Hunt, in fairness, has the same do-not-ask-for-upvotes clause in their FAQ, but given the amount of people blatantly asking for upvotes on Twitter (https://twitter.com/search?q=product%20hunt%20upvote&s=typd) I am doubtful that policy is actually enforced. It's free publicity, after all.
I'm pretty sure this is not true (certainly the method you mention doesn't work), but if you think you know counterexamples, please send them to hn@ycombinator.com. We have put a ton of work into counteracting promotional voting a.k.a. voting rings, and genuine counterexamples (i.e. voting rings that defeated our software) are super valuable to us.
By the way, when pg added that feature years ago, he actually added two features: the colored usernames, plus a page for stories submitted by YC founders. The latter really did have a voting ring effect, and when we figured that out we got rid of it.