With the Phoenix we already have a 'primitive' spacecraft, so there is somewhat of a precedent.
However, I can see two problems:
1) Curiosity is big in the media now. Let's say development of the pet takes two month. Would it still be a major wanted item down the road, when the media coverage has decreased?
2) Pandora's box: When we get a Mars rover, why not the space shuttle Enterprise, too? It has the name, could be a space pet and is canon. Like the ISS, numerous rockets seen in the shows... Where does it stop?
Personally, I think an Sojournerand Enterprise pet remain within reasonable confines. And concerning demand: If the Mars rover pet is a limited time award (like the Phoenix), it would be a boon to the rover fans and item collectors alike.