I'm not really sure it applies to this plugin to be honest. I think of EPUB_ORIGINAL as a second class citizen, it is just something that (sometimes) exists as the result of a conversion (for myself I have it turned off as don't want the functionality). In terms of what will actually get sent to devices etc, it is only the EPUB that is of relevance. Hence that a plugin like Open With only operates on that same "end product" format is something I am quite happy to live with.
The whole point of "ORIGINAL_xxx" as I understand it is that it is for preserving your original format just in case your conversion comes out pants. Hence it should remain untouched, and plugins like Open With shouldn't be working with it anyways.
Thats my 2p anyways