Postachio can embed most of what I need. All the embed codes are here, and note you can use an iFrame directly.
Test This: Workflowy
This is a simple iframe, width is 100%, height is 700, frameborder is 0. I'm using http instead of https for the src URL. Still not sure how to escape the HTML so I can show it to you. Using the entities for less than and greater than doesn't work.
Test This: Hiding
Using simple HTML comments to hide stuff works well!