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DaltonST - I can appreciate that testing your PI's under OS/X or Linux isn't easy without extra hardware and/or perhaps configuring virtual machines.
But Calibre Portable runs on Windows, perhaps you're not aware that it can reside on the same machine as an installed version of Calibre for Windows and they will operate quite independently of one another, different libraries, different configuration data (hence plugins and their data).
The user does NOT have to change any environment settings or jump through any other hoops - all they have to do is to run calibre-portable.exe from wherever they put it.
The only 'limitation' is that you can't run Calibre Portable and Calibre Installed concurrently - but that's because of Calibre's 'single-instance' restriction, nothing to do with Portable per se.
I suggest you 'install' portable and check it out, its a self extracting .exe that unpacks to whatever location you nominate. It could be a thumb drive; anywhere but Program Files or Program Files(x86)
BR