Having had pretty good success running open source software on Windows without having to install it on any particular system I’d be interested in trying out DigiKam if it could be used in the same manner.
My method of running packages in portable fashion involves using a script to start them where the Windows environment variables have been altered to reference the portable disc. This also includes package specific environment variables, if applicable. Registry use is tolerable as long as it is confined to user specific functionality. In that, my definition of portable does not involve taking my portable drive to a computer where someone else might also be running DigiKam.
What is intolerable is either replacing any Windows supplied software or installing any DigiKam specific software in Windows specific folders. It is necessary to take everything that is needed to run DigiKam to another computer where it has NOT been installed and for it to function properly.