It is impossible in Slugline to have Dialogue (or a Dialogue-formatted paragraph) without a Character element right before it.
However, since you can force any line to be a Character element by adding a
@ to the beginning, you can create an "invisible" Character element using a
@ followed by a space. Then press Return and type your Dialogue-formatted text. Here's what that looks like in plain text:
Over black: @ It was a time of great indenting. Some say it was Dialogue, others considered it to be a Title. Still others suggest that it's something in between. All that matters is that you precede it with a "forced" Character element consisting of nothing but a single space. The final frontier.
@_ won't print, so the appreance is that the Dialogue element is unafiliated with a Character.
You can copy/paste the above example right into your Slugline document to try it for yourself.