You’re talking about overriding the formatting engine and breaking how part of markdown is rendered out, so while it doesn’t seem like a big change, it’s not something that can be easily altered, it’s certainly more than a line or two of code.
If you don’t want to change Discourse, then your best bet would be to adopt a different syntax; one which doesn’t collide with Markdown. Have you looked into alternatives? Scriptwriting for example doesn’t prepend lines of dialog with dashes.