The Darth Vader at the Washington Cathedral was the result of a competition for kids to submit drawings of possible grotesques and was highly publicized when it was carved, so I wouldnt necessarily call it an easter egg. But in any case, if anyone is interested, here are a couple other fun ones in the Washington Cathedral that are more easter egg like (put there by the artists of their own accord):

* On the North side, there is a grotesque that portrays a leering stone carver looking down on the girls school across the street, carved by a stonecarver to poke fun at one of his colleagues. Above it, another colleague carved a scolding Dean looking down on the leering carver

* What looks like a stray paint streak on the interior ceiling of the north transept is a rendering of Halley's comet which passed by in '86 when they were painting that ceiling