I stumbled across the following studies which had promising results using Naltrexone on small groups of idiopathic ED sufferers. However, the studies are 15-20 years old.

Endorphins in male impotence: evidence for naltrexone stimulation of erectile activity in patient therapy.
http://www.ncbi.nlm.nih.gov/pubmed/2543996

Treatment of idiopathic erectile dysfunction in men with the opiate antagonist naltrexone–a double-blind study

http://www.andrologyjournal.org/cgi/content/abstract/14/6/407