As it turns out, a Han Solo in Carbonite was brought on set since Nathan Fillion is such a huge Star Wars fan. The props team joked around and added it to sets whenever they could, it apparently became a running joke. It didn't get grabbed out of every scene before shooting though.
Heart of Gold - look just above Mal's left shoulder
Jaynestown - look just to the right of the bartender's head
Objects in Space - look to the left of Book on the counter
The Message - look above Kaylee's hammock on the top right
Serenity - look on Badger's shelf (it looks a bit dusty) - spotted by reader smitsc05
There's a rumor that there's one hidden in every episode, but these are the only ones I could dig up. It makes me extraordinarily happy to know about this hidden gem in the Firefly 'verse.