Itch.io has (as a result of this jam and perhaps a bit of nagging?!) decided to add support for the "monetization" flag on the iframe used to display HTML5 game uploads. This means that from now on HTML5 games can be monetized on itch.io!
Thank you Leaf and the rest of the itch.io staff for making this possible!
/ The Defold Foundation and the Web Monetization community