Is it possible to make vegetarian 'prawn' toast?
This recipe seems to be something close to what you are looking for!
http://vegweb.com/index.php?topic=21443.0
Tofu seems to be a reasonable substitute. 

However you can also find a vegetarian shrimp substitute through this company:
http://www.vegieworld.com/cart/product_pages.asp?id=667

One of their suggested preparation methods is for toast!