Hi, short update:
I created PR which adds OSM iframe whitelisting:
https://github.com/discourse/discourse/pull/2973
If someone is interested I also started development of a simple onebox plugin: discourse-openstreetmap.
It provides oneboxer for links like:
With the recent change to onebox library, oneboxing of links starting with a hash (like OpenStreetMap) works too.
(Perhaps if the PR above gets accepted and the plugin feels mature enough at some point the OSM oneboxer could be introduced as a PR for the onebox library.)