Adobe has always made it a priority to be able to export your entire document in IDML. There's nothing hidden or proprietary in there--just all of the InDesign document content and settings.

In fact, it's standard operating procedures to "cleanse" an .indd file by exporting to IDML and re-importing.

It's up to you to implement something on the receiving end, but there's no lack of openness on Adobe's part.