Ah, solved it: in custom.ini set allow_embedding = true
Just as a request from a user-friendliness-standpoint, it would be cool if features that are disabled in the config would have some sort of notification in the user interface - there is no reason to have the iframe share setting enabled in the user interface if it is disabled in the config. It should be grayed out with a notification that it needs to be enabled by setting allow_embedding = true in the config or something like that. Maybe something for the UX theme to ponder if there is such a team.
# set to true if you want to allow browsers to render Grafana in a <frame>, <iframe>, <embed> or <object>. default is false. allow_embedding = true