Again, I'm not sure how much Steemit (or other apps) actually depend on the format metadata value, so you may get the same results with no format specified at all (because it looks like they preserve sanitized html within markdown when it renders). I noticed after I posted that eSteem provides "markdown+html" as the format value in its posts.

But, because each app renders markdown differently, I would be concerned that doing anything really fancy would result in a post that looks bad when rendered on different apps. eSteem, for instance, does a bad job rendering markdown code blocks (and I'm not even a fan of how Steemit renders code - my own app will use Prism.js to enhance the styling of code blocks).