Hi
The belly cast sounds like a beautiful idea! I had my belly cast 2 days before I gave birth to DD and I have it hanging in her room, waiting to be decorated by her when she gets older but I wish I had thought of having it done earlier for my baby shower.

In regards to food, I don't think its rude to ask people to bring a plate of food at all, especially as I am presuming those getting invited are your nearest and dearest who will probably be happy to help out. For mine, I had all the mums/aunties asking what they could bring and as it was an afternoon thing, we just had plates of cakes/slices and little quiches etc that my mum took charge of in regards to the oven and distribution.

I'm sure your day will be beautiful - all the best!