Technically this should be possible, but it will require a custom trap receiver connector.
The trap receiver could then receive the trap, read the trap binding where the HTML code is present and publish the content to a web server.
A link to the published page could then be included in your visio.
While this will technically work, I don't believe that a trap is the best way to transfer an iFrame. Ideally, you fetch the iframe directly from the source using an HTTP call.