Does anyone know of any undocumented vintage LEGO sets such as Promos or mail in order items? I know of some but it'd be interesting to find out if there were others.
I cannot think of comprehensive resource that would list this kind of details. The only thing on my mind is The Unofficial LEGO Sets/Parts Collectors Guide, which can have some of the information you are looking for.
Gary Istok aka "LEGO Historian" has been known for gathering information on lots of old and obscure LEGO stuff. He has occasionally shared some of this in form of posts on Eurobricks.