Is there a way to use Index cards on the corkboard to store content instead of just a synopsis?
It depends what you mean by ‘store content’. If you write text on to the index cards, it isn’t going to go away! 
You could also put content on to index cards, and then ultimately transfer text from those index cards to the Editor for more extensive manipulation by using the command: Documents > Append Synopsis to Main Text.
H
The synopses are specifically for synopses, though - typing in them will never affect the main text. The latest beta (http://www.literatureandlatte.com/videos/Importing.mov) can be set (via the Preferences) to show the first few lines of the main text if they are empty, as a placeholder, though.
All the best,
Keith