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@Kovid,
Regarding the 'Original' format, a simple solution might be to have a new filetype *.org|*.orig which you can only convert from, not to.
It'll just need a right-click menu in metadata editor to designate which of the formats to set as 'orig', which will then COPY that format and just rename it *.org|*orig
(If needed for conversion process, you can store in *.opf what format the *orig file is in)
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