The objective of the ORIGINAL_XXXX files is to enable speedy recovery to a starting point in the event that a like-to-like conversion, Polish etc produces a poor result. If editing was allowed it would compromise this objective.
You might be able to create Action chain to copy the .ORIGINAL_EPUB into the book's data folder, rename it to .EPUB and open that with the editor. See ==>>
Action Chains plugin.
BR