ok one final attempt before I give up completey
Lenord has a recurring dream, where Pennny is in Sheldons robot.
Even if the actress would have been available, most of the dreams don't last long on the show. So, reoccurring lengthy dreams would be an odd fit for the show.
Also, I feel like you're fixing a problem that wasn't really a big problem. It's not like they had to hide her pregnancy.