Even if I could add more than one keyword by clicking the "Add...>ACDSee Metadata>Keywords" several times, that would help, but currently Pro 8 (and all previous iterations) will allow searching on only one keyword at a time.

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It's just not obvious from the UI. If you add a keyword to the search criteria it starts off with 'is' underlined. If you click that you get a list comparison types (is, is any of, contains, starts with). 'is any of' will return anything containing at least one of the keywords you give it.
Unfortunately the UI doesn't hint at all as to how to specify multiple keywords. You just type them all in to the edit box with either space, comma or semi colon as a separator.