If I were doing Elder Sign or Arkham Horror, I'd probably put both the front and back on the face, possibly with the back upside down so you can represent turning it over by rotating it 180º. I wrote an API script for Keiss a while back that handled that problem for Betrayal at House on the Hill (where the back tells you which part of the house you can play the tile in) by replacing the card with a token when the card was played on the table. You could do something similar with the monster cards, and then have a button to flip them to another side by changing the graphic. (or use a rollable table token for each card and select the face).
