I was wondering if it would be possible to create a way to save comics you are working on, but not quite ready to go put on the site for public scrutiny, so you could edit / publish at a later date. 

At present I tend to screenshot my ideas on the edit screen and pop them in a folder on my computer for later perusal, but it would be nice if there were a way to save them on the site and edit them later instead. 

You can just save them like a normal comic.  Maybe you can add them to a set called "Unfinished" or something.  Then you can use the edit feature go back and change them.

The only drawback is that people can see the unfinished comic in the interim, but that's not a huge deal.

Pen and paper?

I keep several hidden notebooks filled with ideas no one will ever see. It helps me sleep better at night. Trust me, I've felt your pain.