Is it possible to use bespoke markers that do not resemble the "Hiro" marker like below. Could I potentially use a random shape (a solid red oval for example) as a marker?
If this is not currently supported, could someone point me into the right direction to where I might start building this functionality?
AR.js supports custom markers. Make any silly image with the marker generator.
Let ar.js know that you want to use your marker:
<a-marker type="pattern" url="patterns/mypattern.patt">
<a-entity myobject></a-entity>
</a-marker>
and voila. You can check it out in this glitch using this image.