...but it's now experimental...
That's the understatement of the century. It's beginning to dawn on me just how dangerous this option actually is - if combined with auto-organizing it's capable of completely decimating whole albums and sending them in pieces to the four corners of the kingdom. As such, while it's still experimental, I'd make it harder for people to actually use it, like maybe half-disabling the "apply" button for now and just allowing previews. It does seem that setting a hotkey to one does not actually apply the results, though the status-bar says it was applied - this may be unintentional, but it's good for safety. Same caveat would apply to auto-applying - not sure if it works or not, but might be wise to disable that too, just in case.
The "Always preserve these tag values" list is invaluable, to be sure. If you decide to keep this <All Tags> option permanently, there might need to be a similar list where it's "only apply to" instead, as sometimes the "all" in "all tags" is a bit too many.
Also, unrelated observation, in the description for "Always preserve" it says to separate the values with ";;" but using two semi-colons actually breaks the list (as in only the first item is preserved, the rest are applied). It only works as described when using a single semi-colon to separate the list. You might change that label.
And as Boroda didn't make this clear above, I'll say it: As a warning to anyone else experimenting with this, I can't underscore enough just how unintentionally destructive a command it can be by it's very nature. Do not mess around with it on part of your living library - for now, only apply it to albums/songs you have no need for.I shall experiment more, obviously. I had a few crashes at first, so it's not entirely stable (but I guess you know that). I didn't want to report the error logs until I can actually reproduce the steps necessary to reliably crash it again.
Thanks for your efforts. (The idea seemed harmless enough originally... I'm rethinking that a bit now.)