"That would render us mercenary and contemptible individuals of great weakness of character. It cannot be the right thing for us to do," he said.
The other backers of Assange include retired professor Tricia David, Nobel Prize-winning scientist John Sulston, journalist Philip Knightley, Lady Caroline Evans, friend Sarah Saunders, Joseph Farrell, Sarah Harrison and Tracy Worcester.
Riddle said he has a "fair amount to think about" and will make a decision in a few days.
Talks between British and Ecuadorean officials to resolve the deadlock over Assange's fate have not been fruitful. British officials say Assange will be arrested if he steps outside the embassy.
Cassandra Vinograd can be reached at htpp://twitter.com/CassVinograd