Yes. For example, I put a previous command at the prompt to be modified. It began with 'insert ...' so I added an initial \e to the command. psql told me that \einsert is not a valid command after I pressed the [Enter] key.
You type "insert", realize you want an editor for this, hit enter (multi-line mode is psql), type \e, hit enter again, your editor appears with "insert" already in place from the query buffer. Upon returning you are given a new buffer with the contents of whatever you typed into the editor pasted in.