] ] It appears that gambling debts (debts of honor) were enforceable, but I'm not sure how. Could the debtor be sued in court? Was the penalty "only" disgrace? Was failure to pay grounds for a duel?

This is still not exactly an answer to the above, but it makes things a little clearer. Emerson relates a story of the eminent Georgian Whig, Charles James Fox which sheds some light on the concept of a 'debt of honour.' From what I can gather, the term encompasses any loans or gambling debts which have not been documented. Thus if the debtor dies, the lender has no way of regaining his/her money because nothing has been written down. Many considered their debts of honour to take precedence over the other sort, as Emerson's story about Fox reveals:

A tradesman who had long dunned him [Fox] for a note of three hundred guineas, found him one day counting gold, and demanded payment. "No," said Fox, "I owe this money to Sheridan: it is a debt of honour: if an accident should happen to me, he has nothing to show."

"Then," said the creditor, "I change my debt into a debt of honour," and tore the note in pieces. Fox thanked the man for his confidence, and paid him, saying, "his debt was of older standing, and Sheridan must wait."

Another Charles Fox anecdote which I found online, concerns Fox's grandparents, who were evidently married to settle a debt of honour!

Fox's grandmother, Sarah Cadogan (His mother's mother) was 13 when she was married to the 18 year old Charles, later the second Duke of Richmond to settle a gaming debt between their fathers. The pair had disliked each other on sight and after the wedding she was packed off to school and he was packed off to the continent for the Grand Tour.

Charles returned three years later and on his first night back hoping to delay or just plain avoid the inevitable meeting with his loathed wife he went to the opera. In the a box opposite he spied a most beautiful woman, whom he fell in love with. It turned out this was his wife and he wooed her properly this time and successfully. Their marriage was universally regarded as happy and they were known to kiss, coo and cuddle constantly. She fell pregnant to him 28 times during their long marriage, and had 12 children.