I have a lot of Italian picture references for cod pieces, undies and the likes. Here's the most interesting ones. In the links, the name of the artist is written towards the end, along with year/decade, and eventual clues on title and/or location.

Red breeches, doublet and cod piece under black overgarment:
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/menlombardy/moroni1559soldiermadrid.jpg

Cod piece o'hoy!
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/menlombardy/moroni1565antnavageromilan.jpg

Not the bum as such, but... (A Fasolo fresco from Villa Caldogno near Vicenza, Italy)
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/venezia2/detail2.jpg

Rare portrait of a man in shirt and breeches (from Villa Chiericati in Vicenza):
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/venezia2/chiericati9.jpg

Another cod piece o'hoy:
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/menemiliaromagna/parmagianinopiermariarossi1prado.jpg

From Statens Museum for Kunst in Copenhagen:
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/mentuscany/coassone15thctrouserscopenhagen.jpg
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/copenhagen/museum2.jpg

Lots of undies here...
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/mentuscany/macchietti1570spalazzovecchio.jpg

Farmers working in the field, from the Torre Aquila in Trent, Italy:
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/temporary/buonconsigliotrent1spring.jpg

From Piero della Francesca's "Legend of the true cross" (I mislabeled it):
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/siena/1440shospitalsiena8.jpg

From the former hospital of Santa Maria della Scala in Siena, Italy:
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/siena/1440shospitalsiena6.jpg
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/siena/1440shospitalsiena3.jpg

From Palazzo Schifanoia in Ferrara, Italy:
http://img.photobucket.com/albums/v31/operafantomet/renaissanceportraits/ferrara/schifanoia1470ferrara2.jpg