TidyPas is just a wrapper around the HTML Tidy library API. It does not provide a UnicodeString facade over that API, it exposes the API as-is.
As far as I can tell from the docs, HTML Tidy itself only supports a limited range of character sets, but these do include the UTF8 encoding of Unicode, which with a bit of care I think should be OK with ANSIString and ANSIChar types used by the API.
Any further inquiries about Unicode support in HTML Tidy other than with UTF8 would probably be best directed at the HTML Tidy community itself. It doesn't seem to have been updated for a while though (since 2008).