We use the diaper shower quite a bit. I work in an elementary school - lots of young mamas there. For a second baby it's traditional around here to NOT have a shower. But that just doesn't feel right! When we want to acknowledge the little one without making a big deal we do a diaper shower. Or if it's one of our men we know there will be a shower where the mom works and/or at church. To gift them easily without overwhelming them with Size 0 - 3 month clothes we do a diaper shower. Everyone brings a package of diapers (the mom-to-be is asked which brand is preferred) or wipes or lotions, etc. Budreaux's Butt Paste is a favorite gift at such things. It's great for diaper rash. It's an unwritten rule to aim at larger sizes. THere will also be gift cards for Wal-mart or the grocery store. The honoree can take back whatever she has too much of and get things to fill in the gaps.
You get to gift the family without it being a big deal. Just enough to say "welcome!" That's why showing up with such a bigger gift may not be right.