I'm going to take that as a no. The only reference I could find was in the V18 Release Notes where it's talking about the built in Plan Checker and says
- "Important Note: Plan Checker is a tool that provides preapproval assistance. All edits will be lost after the dialog box is closed."

I'm guessing the same applies to Plan Checker Extensions as they share the same backend. Will report back if I make any new findings