We do not provide embed codes at the moment but it is definitely something I will pass on to the team for a potential future enhancement. As you say there are no customisation options in the free to use Starter system but the Starter Plus upgrade does include customisation of the public portal.
I would like to embed the public access portal into our organization's website, so that viewers will know what organization the documents belong to (since the Starter edition doesn't offer customization). However, it seems like there are settings preventing me from creating an iFrame. Do you offer embed codes?