Silk Cloth Poster

You can find an assortment of hanging clips at the site below, and some of them use the wall-safe 'Command' adhesive system. It sounds like the mini-hangers are the right size for what you're doing. These clips are all archivally safe (designed for quilt display originally) and won't hurt your poster at all, so if 'Command' adhesive products work on your wall, you should be in business. (Please note that Fun-Tac or any of the gum-like products are likely to stain the silk, and aren't considered archivally safe.)

http://www.hangupscompany.com/wooden_hangers.html

Good luck!