I have not tried doing this but I guess that there are a couple of problems with using a greenhouse to dry wood. Firstly, it would be very difficult to regulate the temperature. If it was too hot the wood may overheat, dry quickly and split. Secondly, air circulation. A drying kiln needs air movement to work correctly, a fan or similar. Even air dried wood still needs the air to circulate.
Having said all that, you are talking about pen blanks, small lumps of wood. I think it must be worth having a go with these as long as the temperature is controlled by sheeting or something similar.
This is only my opinion, somebody with a lot more knowledge may tell you something different.
Bob