Can I log any PIC time if my medical certificate is out of date and I haven't flown in 20 years?
To answer your question, specific to the US, you may log your fam flight with a CFI as dual received, and also as PIC for the time you are the sole manipulator of the controls (which will likely be the entire flight).

You don't need a medical to log PIC, but you will likely need one to act as PIC, depending upon what you are flying.  

But for training, you clearly can log time as PIC when you are the sole manipulator of the controls, in a class and category for which you are rated.

Have fun!