I think you need to just think very hard about why you are having the party and word the invitation that way. If you are celebrating your pregnancy then it is not really a baby shower (because that would be "showering" you with gifts for the "baby") so you need to make it clear that it is not a shower by not having that word or anything like it on the invitation. Please understand that loved ones WILL want to buy you a gift, so either accept that fact or do like someone else suggested and put on the invitation that you will be accepting either monetary or gift donations for a local charity, like teen moms or a homeless shelter or something similar.
I know this sounds kooky, but when I was reading your post, I thought about an episode of friends when Phoebe was prenant as a serogate so they threw a shower and gave her gifts for things that would be for HER instead of for the baby. Even though you say you have clothes for this baby, you can still register at Babys R Us for stuff you and the new baby will need or want, like new baby washcloths, a new sling, a new diaper bag, a new boppy pillow, new baby blankets, new hoodie towel, etc. Seriously, I do think that there are more people in your life who will want to get you a little something than people who think it's tacky to throw a shower for a second child. Chances are that your friends and family WILL buy you something for the baby and you'll end up having a lot of stuff to return if you don't register for what you want. You can word it somehow that you want donations for a charity but mention that you are also registered at Babys R Us in small print by saying while they are there picking up a gift for the charity, gifts for you are not required but you registered for a few small things "just in case"! You know the people on your guest list better than I do-- the friends and family at the shower for my 3rd child really loved getting all of those little new things like the stuff I mentioned above, plus a new little outfit just for the fun of shopping for it!