Interesting turn in the discussion. If people want to have a party to celebrate subsequent babies, and even call it a shower, maybe that will eventually become totally okay, and guests won't automatically associate a shower with a gift-giving occasion. It can be, "We're having a shower for Susie's third baby," and Susie doesn't have a prominent registry, and guests just bring a box of wipes or a package of diapers or even just a card. (I think at least one PP said this was how showers for subsequent babies were already done in her circle.)
I wouldn't disapprove of such a thing. I happen to not care for parties much in general so I don't know that I'd attend, but I would see it as basically on the same level as someone's first backyard BBQ of the season--some people just like to socialize, any reason will do, and it's fun to celebrate a new (upcoming) baby.
Unfortunately, in my personal experience, this hasn't come to pass yet. I see showers for subsequent babies where the host or GOH are expecting gifts from everyone on a "first shower ever" level, or maybe just a touch lower, with a full registry made available. I haven't actually been invited to one yet, as they aren't happening to anyone I really know, but more friends of friends.
Maybe one thing to do is attend these showers with the former attitude in mind, that you're there to cheerfully celebrate the new baby, and bring just a small gift. (Like when my friend Amy got married, she had two bridal showers, and I was invited to both. The first one I bought her a nice gift. The second one, I wrote a nice message in a card for her. Another friend who was invited to both fretted about having to get yet another gift for the second shower, and ended up anxious and resentful.) Maybe that will catch on with people.