In a game earlier today I used Overlord as a Ratcatcher and then another Overlord as a Wine Merchant. The Overlord does appear to go to the reserve mat, but cannot be called back.
As a Ratcatcher, the Overlord went to my reserve mat, but on subsequent turns there is no option to call him back.
As a Wine Merchant, the Overlord went to my reserve mat, but was not called back when I underpaid for a card.
Just FYI
This is not a bug: Overlord (and Band of Misfits) are only the respective card until they leave play. Therefore, once Overlord is on the reserve mat, it's Overlord again and doesn't have the Ratcatcher / Wine Merchant's abilities to be called.
Interesting. If cards on the reserve mat are not 'in play', what are they? Are they just considered 'In reserve'?
Yes.
Also an Island and the card you set aside with it is not in play anymore. You can also find an explanation of the interatcion between reserve card and Band of Misfits here http://wiki.dominionstrategy.com/index.php/Band_of_Misfits