I think it would be very handy if there was some sort of a way to create a static item in k2 which always displays in let's say category item view or on the homepage when all the items are listed.It would be great for advertisements and stuff. Anyone any idea on how to do this?

Go to modules. Click create new. And have a close look at various k2 module options, especially K2 Tools. Perhaps you will find something that might suit your needs.