Problem is your original crossover idea seemed limited to -oddly- series' shows that appeared on another series' DVD set. We've taken that idea and expanded it to something more worthy of discussion.
I agree that Kramer's apperance on MAY was a "gimmick" but it also intended -and did- establish that MAY and Seinfeld took place "in the same universe."
And, yes, as I said we see George fret over watching MAY with Susan in one episode but such a thing doesn't preclude the Buchmans living in Seinfeld's New York it just means that a show called MAY exsists in Seinfeld's and MAY's universe though it is very likely very different than the MAY you and I know. And as I pointed out all of these other "gimmicks" firmly links almost all of NBC's "New York Sitcoms" to one another. Friends and Mad About You were linked from early in the first season with Ursala and she remained a recurring character throughout the show's run. So at the very least it's cemented that MAY and Friends are "in the same universe."
It's just more fun to link shows in this manner than to talk about what show appeared in another's DVD set. Because such an occurance doesn't really mark the episode as being part of two series as doing such a thing is really a marketing gimmick.