You're right, it's very tacky.
Worse, though, is telling someone "Wow, I'm not going to participate in something so rude."
Just say you're very happy for "Jennifer" and can't wait to meet the new baby, and you won't be available for a shower.
Or, if you wish, say you'd love to co-host a NO GIFTS tea or luncheon. That's a perfectly acceptable, lovely way to spend some time together without being so tacky as to have a shower for a second-time Mom.
I agree with RoxyLynn. Where I'm from, showers simply aren't done for second (and subsequent) children. You only become a mother once, with the first, and that's what a shower celebrates...babies themselves are celebrated with christenings/baptisms/namings/meet the baby parties.
I'd simply decline by saying you aren't available to help with the shower. Or, you could suggest a no gifts luncheon (although saying no gifts on the invitation is, itself, a violation of etiquette).