People on top of the cake is actually not done in a lot of circles (here comes the bad pun! ) so use an object- my thought is a pair of shiny 7005 rear cogs linked with a peice of chain wrapped around. Lots of appropriate symbolism, something different, and you can nail it up in the house as art after. Put a lot of pretty flowers around it.
If you're doing the church thing, I just rounded up a bunch of chainrings and goodies to make bike girl tiaras. Yes I'm that tiara freak as well as a (semi) working artist. When are you doing this?
If you're doing this anywhere else, round up a bunch of old frames or whole bikes on the cheap (or free!) and have a freind who welds tack them into an arch; apply flowers. There's a giant bike arch in, I think it's Durango CO where all the riders live. Look for a picture. Anyway, you would get married under it instead of the usual trellis thingy. And you can get all white bike clothes. Course you may save clomping around in cleats for the vow renewal... Your mama and them have expectations.
Call up Susan at our gracious forum hosts TE and see if she has a line on cute bike crap. The people at Terry might help- they have a ton of neat stuff and may know where more comes from.
Will be back with more- I'm already making a chandelier out of brake levers, the juices are flowing...

Lizzy