What can I use to rigidly hang a lot of flat/paper artwork on a wall?
You could try painting it with a magnetic paint and use those strong small earth magnetics to hold the art up. The magnetic wall paint can be top-coated with a single coat of whatever colour you want.

There are a number of paints from different manufactures to choose from, some with better reviews than others. It seems multiple coats are needed, but other people report success. Strong magnets are a must.

I have not used this myself, but I have seen it used in a number of design magazines, etc.