I thought a general rule was to add -en to the end of any Masculine nouns referring to people in cases other than nominative. Is that wrong, or is this an exception?
You're thinking of "weak" nouns, and it's more complicated than that. See: http://german.about.com/od/grammar/a/dernouns.htm
There's a glossary on the second page of that article.
I've never heard of that rule. Only changes to the noun I know of are to add -s/-es to the end of masculine and neutral nouns in the genetiv and change the noun slightly in the plural dativ.