What are the limitations that don't let you embed a Marketo form? Can you send me an email with some details about the issue you're running into so that we can consider it as a future forms improvement?
AMP is kind of an interesting case, because Marketo really needs to do two things here: integrate and align Munchkin tracking with amp-analytics and use amp-embed to render the forms themselves.
You could get around this today (sans tracking) if you really wanted to via amp-iframe and defined sizes, for what it's worth.